* Re: hdmi-codec: modifying params in hw_params() callback
@ 2019-02-28 14:35 ` Jyri Sarha
0 siblings, 0 replies; 17+ messages in thread
From: Jyri Sarha @ 2019-02-28 14:35 UTC (permalink / raw)
To: Russell King - ARM Linux admin
Cc: Liam Girdwood, Mark Brown, alsa-devel, linux-arm-kernel,
linux-kernel
On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
> On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
>> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
>>> Hi all,
>>>
>>> While looking at hdmi-codec issues, I spotted this code:
>>>
>>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
>>> struct snd_pcm_hw_params *params,
>>> struct snd_soc_dai *dai)
>>> {
>>> ...
>>> if (params_width(params) > 24)
>>> params->msbits = 24;
>>>
>>> params->msbits is a parameter that is negotiated and refined by
>>> libasound, and is part of the ALSA constraint system. The "Writing an
>>> ALSA driver" documentation says about the hw_params callback:
>>>
>>> "This is called when the hardware parameter (``hw_params``) is set up
>>> by the application, that is, once when the buffer size, the period
>>> size, the format, etc. are defined for the pcm substream."
>>>
>>> which suggests we should only be reading the parameters, not writing
>>> to them.
>>>
>>> What's more is that the msbits is a parameter that is refined with
>>> userspace, so surely the above should be a declared constraint?
>>>
>>
>> Certainly not a constraint. While HDMI can only pass up to 24-bit per
>> sample audio the most (or at least the two I have worked with) encoders
>> can take 32-bit wide (and probably wider) samples trough i2s. So the
>> system can play 32-bit samples, just the 4 LSB is ignored.
>
> You have not explained why "not a constraint". From the explanation
> that Mark gave me on IRC, msbits is the number of bits that the codec
> respects, whereas the sample bits is the number of bits in the sample.
> If sample bits - msbits != 0, then that is the number of bits ignored.
> Mark basically described it as "feel free to send me more than 24 bits
> but I'm ignoring the LSBs".
>
> If the msbits is limited to 24, then userspace gets to know about that
> limitation when negotiating with ALSA if it is a constraint. If it
> isn't a constraint, userspace has no way to know.
>
>> Actually there is very little difference between the case of playing
>> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
>> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
>> ignored like in HDMI audio case.
>
> You mean 8 LSB throughout the above two paragraphs.
>
>>> Digging a bit deeper, ASoC passes a private copy of the params to each
>>> codec - a copy is made, then fixups for TDM slots are applied, followed
>>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
>>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
>>> back the rate, channels and physical (memory) width and stores them
>>> in the codec's DAI structure. The msbits are not read.
>>>
>>> hdmi-codec also seems to do nothing with the msbits parameter other
>>> than the above code.
>>>
>>> So, all in all, it seems that the above code limiting msbits is
>>> redundant - nothing will read this modified value. Can we kill it?
>>>
>>
>> It certainly looks that way, so yes. In any case struct
>> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
>
> This sets a constraint to limit msbits to 24 for any sample width
> in core ASoC code, doing exactly what you said "certainly not" to
> above.
>
> It also means that the quoted code in hdmi-codec's hw_params() is
> redundant for another reason.
>
> See the description of:
> snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
> soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
>
> Can you please clarify, because there seems to be some confusion.
>
If I understand right, the only confusion here is around the term
"constraint". I did not follow that you meant in strictly alsa-term. Yes
we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
does), because HDMI interface can not deliver more than 24 bits per
sample, and any extra LSB bit will be ignored.
--
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
@ 2019-02-28 14:35 ` Jyri Sarha
0 siblings, 0 replies; 17+ messages in thread
From: Jyri Sarha @ 2019-02-28 14:35 UTC (permalink / raw)
To: Russell King - ARM Linux admin
Cc: alsa-devel, Mark Brown, Liam Girdwood, linux-arm-kernel,
linux-kernel
On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
> On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
>> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
>>> Hi all,
>>>
>>> While looking at hdmi-codec issues, I spotted this code:
>>>
>>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
>>> struct snd_pcm_hw_params *params,
>>> struct snd_soc_dai *dai)
>>> {
>>> ...
>>> if (params_width(params) > 24)
>>> params->msbits = 24;
>>>
>>> params->msbits is a parameter that is negotiated and refined by
>>> libasound, and is part of the ALSA constraint system. The "Writing an
>>> ALSA driver" documentation says about the hw_params callback:
>>>
>>> "This is called when the hardware parameter (``hw_params``) is set up
>>> by the application, that is, once when the buffer size, the period
>>> size, the format, etc. are defined for the pcm substream."
>>>
>>> which suggests we should only be reading the parameters, not writing
>>> to them.
>>>
>>> What's more is that the msbits is a parameter that is refined with
>>> userspace, so surely the above should be a declared constraint?
>>>
>>
>> Certainly not a constraint. While HDMI can only pass up to 24-bit per
>> sample audio the most (or at least the two I have worked with) encoders
>> can take 32-bit wide (and probably wider) samples trough i2s. So the
>> system can play 32-bit samples, just the 4 LSB is ignored.
>
> You have not explained why "not a constraint". From the explanation
> that Mark gave me on IRC, msbits is the number of bits that the codec
> respects, whereas the sample bits is the number of bits in the sample.
> If sample bits - msbits != 0, then that is the number of bits ignored.
> Mark basically described it as "feel free to send me more than 24 bits
> but I'm ignoring the LSBs".
>
> If the msbits is limited to 24, then userspace gets to know about that
> limitation when negotiating with ALSA if it is a constraint. If it
> isn't a constraint, userspace has no way to know.
>
>> Actually there is very little difference between the case of playing
>> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
>> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
>> ignored like in HDMI audio case.
>
> You mean 8 LSB throughout the above two paragraphs.
>
>>> Digging a bit deeper, ASoC passes a private copy of the params to each
>>> codec - a copy is made, then fixups for TDM slots are applied, followed
>>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
>>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
>>> back the rate, channels and physical (memory) width and stores them
>>> in the codec's DAI structure. The msbits are not read.
>>>
>>> hdmi-codec also seems to do nothing with the msbits parameter other
>>> than the above code.
>>>
>>> So, all in all, it seems that the above code limiting msbits is
>>> redundant - nothing will read this modified value. Can we kill it?
>>>
>>
>> It certainly looks that way, so yes. In any case struct
>> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
>
> This sets a constraint to limit msbits to 24 for any sample width
> in core ASoC code, doing exactly what you said "certainly not" to
> above.
>
> It also means that the quoted code in hdmi-codec's hw_params() is
> redundant for another reason.
>
> See the description of:
> snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
> soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
>
> Can you please clarify, because there seems to be some confusion.
>
If I understand right, the only confusion here is around the term
"constraint". I did not follow that you meant in strictly alsa-term. Yes
we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
does), because HDMI interface can not deliver more than 24 bits per
sample, and any extra LSB bit will be ignored.
--
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
2019-02-28 14:35 ` Jyri Sarha
@ 2019-02-28 14:39 ` Russell King - ARM Linux admin
-1 siblings, 0 replies; 17+ messages in thread
From: Russell King - ARM Linux admin @ 2019-02-28 14:39 UTC (permalink / raw)
To: Jyri Sarha
Cc: Liam Girdwood, Mark Brown, alsa-devel, linux-arm-kernel,
linux-kernel
On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
> > On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
> >> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
> >>> Hi all,
> >>>
> >>> While looking at hdmi-codec issues, I spotted this code:
> >>>
> >>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
> >>> struct snd_pcm_hw_params *params,
> >>> struct snd_soc_dai *dai)
> >>> {
> >>> ...
> >>> if (params_width(params) > 24)
> >>> params->msbits = 24;
> >>>
> >>> params->msbits is a parameter that is negotiated and refined by
> >>> libasound, and is part of the ALSA constraint system. The "Writing an
> >>> ALSA driver" documentation says about the hw_params callback:
> >>>
> >>> "This is called when the hardware parameter (``hw_params``) is set up
> >>> by the application, that is, once when the buffer size, the period
> >>> size, the format, etc. are defined for the pcm substream."
> >>>
> >>> which suggests we should only be reading the parameters, not writing
> >>> to them.
> >>>
> >>> What's more is that the msbits is a parameter that is refined with
> >>> userspace, so surely the above should be a declared constraint?
> >>>
> >>
> >> Certainly not a constraint. While HDMI can only pass up to 24-bit per
> >> sample audio the most (or at least the two I have worked with) encoders
> >> can take 32-bit wide (and probably wider) samples trough i2s. So the
> >> system can play 32-bit samples, just the 4 LSB is ignored.
> >
> > You have not explained why "not a constraint". From the explanation
> > that Mark gave me on IRC, msbits is the number of bits that the codec
> > respects, whereas the sample bits is the number of bits in the sample.
> > If sample bits - msbits != 0, then that is the number of bits ignored.
> > Mark basically described it as "feel free to send me more than 24 bits
> > but I'm ignoring the LSBs".
> >
> > If the msbits is limited to 24, then userspace gets to know about that
> > limitation when negotiating with ALSA if it is a constraint. If it
> > isn't a constraint, userspace has no way to know.
> >
> >> Actually there is very little difference between the case of playing
> >> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
> >> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
> >> ignored like in HDMI audio case.
> >
> > You mean 8 LSB throughout the above two paragraphs.
> >
> >>> Digging a bit deeper, ASoC passes a private copy of the params to each
> >>> codec - a copy is made, then fixups for TDM slots are applied, followed
> >>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
> >>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
> >>> back the rate, channels and physical (memory) width and stores them
> >>> in the codec's DAI structure. The msbits are not read.
> >>>
> >>> hdmi-codec also seems to do nothing with the msbits parameter other
> >>> than the above code.
> >>>
> >>> So, all in all, it seems that the above code limiting msbits is
> >>> redundant - nothing will read this modified value. Can we kill it?
> >>>
> >>
> >> It certainly looks that way, so yes. In any case struct
> >> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
> >
> > This sets a constraint to limit msbits to 24 for any sample width
> > in core ASoC code, doing exactly what you said "certainly not" to
> > above.
> >
> > It also means that the quoted code in hdmi-codec's hw_params() is
> > redundant for another reason.
> >
> > See the description of:
> > snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
> > soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
> >
> > Can you please clarify, because there seems to be some confusion.
> >
>
> If I understand right, the only confusion here is around the term
> "constraint". I did not follow that you meant in strictly alsa-term. Yes
> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
> does), because HDMI interface can not deliver more than 24 bits per
> sample, and any extra LSB bit will be ignored.
Right, so we can remove the above code then?
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
@ 2019-02-28 14:39 ` Russell King - ARM Linux admin
0 siblings, 0 replies; 17+ messages in thread
From: Russell King - ARM Linux admin @ 2019-02-28 14:39 UTC (permalink / raw)
To: Jyri Sarha
Cc: alsa-devel, Mark Brown, Liam Girdwood, linux-arm-kernel,
linux-kernel
On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
> > On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
> >> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
> >>> Hi all,
> >>>
> >>> While looking at hdmi-codec issues, I spotted this code:
> >>>
> >>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
> >>> struct snd_pcm_hw_params *params,
> >>> struct snd_soc_dai *dai)
> >>> {
> >>> ...
> >>> if (params_width(params) > 24)
> >>> params->msbits = 24;
> >>>
> >>> params->msbits is a parameter that is negotiated and refined by
> >>> libasound, and is part of the ALSA constraint system. The "Writing an
> >>> ALSA driver" documentation says about the hw_params callback:
> >>>
> >>> "This is called when the hardware parameter (``hw_params``) is set up
> >>> by the application, that is, once when the buffer size, the period
> >>> size, the format, etc. are defined for the pcm substream."
> >>>
> >>> which suggests we should only be reading the parameters, not writing
> >>> to them.
> >>>
> >>> What's more is that the msbits is a parameter that is refined with
> >>> userspace, so surely the above should be a declared constraint?
> >>>
> >>
> >> Certainly not a constraint. While HDMI can only pass up to 24-bit per
> >> sample audio the most (or at least the two I have worked with) encoders
> >> can take 32-bit wide (and probably wider) samples trough i2s. So the
> >> system can play 32-bit samples, just the 4 LSB is ignored.
> >
> > You have not explained why "not a constraint". From the explanation
> > that Mark gave me on IRC, msbits is the number of bits that the codec
> > respects, whereas the sample bits is the number of bits in the sample.
> > If sample bits - msbits != 0, then that is the number of bits ignored.
> > Mark basically described it as "feel free to send me more than 24 bits
> > but I'm ignoring the LSBs".
> >
> > If the msbits is limited to 24, then userspace gets to know about that
> > limitation when negotiating with ALSA if it is a constraint. If it
> > isn't a constraint, userspace has no way to know.
> >
> >> Actually there is very little difference between the case of playing
> >> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
> >> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
> >> ignored like in HDMI audio case.
> >
> > You mean 8 LSB throughout the above two paragraphs.
> >
> >>> Digging a bit deeper, ASoC passes a private copy of the params to each
> >>> codec - a copy is made, then fixups for TDM slots are applied, followed
> >>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
> >>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
> >>> back the rate, channels and physical (memory) width and stores them
> >>> in the codec's DAI structure. The msbits are not read.
> >>>
> >>> hdmi-codec also seems to do nothing with the msbits parameter other
> >>> than the above code.
> >>>
> >>> So, all in all, it seems that the above code limiting msbits is
> >>> redundant - nothing will read this modified value. Can we kill it?
> >>>
> >>
> >> It certainly looks that way, so yes. In any case struct
> >> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
> >
> > This sets a constraint to limit msbits to 24 for any sample width
> > in core ASoC code, doing exactly what you said "certainly not" to
> > above.
> >
> > It also means that the quoted code in hdmi-codec's hw_params() is
> > redundant for another reason.
> >
> > See the description of:
> > snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
> > soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
> >
> > Can you please clarify, because there seems to be some confusion.
> >
>
> If I understand right, the only confusion here is around the term
> "constraint". I did not follow that you meant in strictly alsa-term. Yes
> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
> does), because HDMI interface can not deliver more than 24 bits per
> sample, and any extra LSB bit will be ignored.
Right, so we can remove the above code then?
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
2019-02-28 14:39 ` Russell King - ARM Linux admin
(?)
@ 2019-02-28 14:41 ` Jyri Sarha
-1 siblings, 0 replies; 17+ messages in thread
From: Jyri Sarha @ 2019-02-28 14:41 UTC (permalink / raw)
To: Russell King - ARM Linux admin
Cc: Liam Girdwood, Mark Brown, alsa-devel, linux-arm-kernel,
linux-kernel
On 28/02/2019 16:39, Russell King - ARM Linux admin wrote:
> On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
>> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
>>> On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
>>>> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
>>>>> Hi all,
>>>>>
>>>>> While looking at hdmi-codec issues, I spotted this code:
>>>>>
>>>>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
>>>>> struct snd_pcm_hw_params *params,
>>>>> struct snd_soc_dai *dai)
>>>>> {
>>>>> ...
>>>>> if (params_width(params) > 24)
>>>>> params->msbits = 24;
>>>>>
>>>>> params->msbits is a parameter that is negotiated and refined by
>>>>> libasound, and is part of the ALSA constraint system. The "Writing an
>>>>> ALSA driver" documentation says about the hw_params callback:
>>>>>
>>>>> "This is called when the hardware parameter (``hw_params``) is set up
>>>>> by the application, that is, once when the buffer size, the period
>>>>> size, the format, etc. are defined for the pcm substream."
>>>>>
>>>>> which suggests we should only be reading the parameters, not writing
>>>>> to them.
>>>>>
>>>>> What's more is that the msbits is a parameter that is refined with
>>>>> userspace, so surely the above should be a declared constraint?
>>>>>
>>>>
>>>> Certainly not a constraint. While HDMI can only pass up to 24-bit per
>>>> sample audio the most (or at least the two I have worked with) encoders
>>>> can take 32-bit wide (and probably wider) samples trough i2s. So the
>>>> system can play 32-bit samples, just the 4 LSB is ignored.
>>>
>>> You have not explained why "not a constraint". From the explanation
>>> that Mark gave me on IRC, msbits is the number of bits that the codec
>>> respects, whereas the sample bits is the number of bits in the sample.
>>> If sample bits - msbits != 0, then that is the number of bits ignored.
>>> Mark basically described it as "feel free to send me more than 24 bits
>>> but I'm ignoring the LSBs".
>>>
>>> If the msbits is limited to 24, then userspace gets to know about that
>>> limitation when negotiating with ALSA if it is a constraint. If it
>>> isn't a constraint, userspace has no way to know.
>>>
>>>> Actually there is very little difference between the case of playing
>>>> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
>>>> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
>>>> ignored like in HDMI audio case.
>>>
>>> You mean 8 LSB throughout the above two paragraphs.
>>>
>>>>> Digging a bit deeper, ASoC passes a private copy of the params to each
>>>>> codec - a copy is made, then fixups for TDM slots are applied, followed
>>>>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
>>>>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
>>>>> back the rate, channels and physical (memory) width and stores them
>>>>> in the codec's DAI structure. The msbits are not read.
>>>>>
>>>>> hdmi-codec also seems to do nothing with the msbits parameter other
>>>>> than the above code.
>>>>>
>>>>> So, all in all, it seems that the above code limiting msbits is
>>>>> redundant - nothing will read this modified value. Can we kill it?
>>>>>
>>>>
>>>> It certainly looks that way, so yes. In any case struct
>>>> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
>>>
>>> This sets a constraint to limit msbits to 24 for any sample width
>>> in core ASoC code, doing exactly what you said "certainly not" to
>>> above.
>>>
>>> It also means that the quoted code in hdmi-codec's hw_params() is
>>> redundant for another reason.
>>>
>>> See the description of:
>>> snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
>>> soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
>>>
>>> Can you please clarify, because there seems to be some confusion.
>>>
>>
>> If I understand right, the only confusion here is around the term
>> "constraint". I did not follow that you meant in strictly alsa-term. Yes
>> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
>> does), because HDMI interface can not deliver more than 24 bits per
>> sample, and any extra LSB bit will be ignored.
>
> Right, so we can remove the above code then?
>
Yes, you correctly pointed out that this piece of code does nothing and
is conceptually wrong:
if (params_width(params) > 24)
params->msbits = 24;
Yes. It can be removed.
--
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
@ 2019-02-28 14:41 ` Jyri Sarha
0 siblings, 0 replies; 17+ messages in thread
From: Jyri Sarha @ 2019-02-28 14:41 UTC (permalink / raw)
To: Russell King - ARM Linux admin
Cc: Liam Girdwood, Mark Brown, alsa-devel, linux-arm-kernel,
linux-kernel
On 28/02/2019 16:39, Russell King - ARM Linux admin wrote:
> On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
>> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
>>> On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
>>>> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
>>>>> Hi all,
>>>>>
>>>>> While looking at hdmi-codec issues, I spotted this code:
>>>>>
>>>>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
>>>>> struct snd_pcm_hw_params *params,
>>>>> struct snd_soc_dai *dai)
>>>>> {
>>>>> ...
>>>>> if (params_width(params) > 24)
>>>>> params->msbits = 24;
>>>>>
>>>>> params->msbits is a parameter that is negotiated and refined by
>>>>> libasound, and is part of the ALSA constraint system. The "Writing an
>>>>> ALSA driver" documentation says about the hw_params callback:
>>>>>
>>>>> "This is called when the hardware parameter (``hw_params``) is set up
>>>>> by the application, that is, once when the buffer size, the period
>>>>> size, the format, etc. are defined for the pcm substream."
>>>>>
>>>>> which suggests we should only be reading the parameters, not writing
>>>>> to them.
>>>>>
>>>>> What's more is that the msbits is a parameter that is refined with
>>>>> userspace, so surely the above should be a declared constraint?
>>>>>
>>>>
>>>> Certainly not a constraint. While HDMI can only pass up to 24-bit per
>>>> sample audio the most (or at least the two I have worked with) encoders
>>>> can take 32-bit wide (and probably wider) samples trough i2s. So the
>>>> system can play 32-bit samples, just the 4 LSB is ignored.
>>>
>>> You have not explained why "not a constraint". From the explanation
>>> that Mark gave me on IRC, msbits is the number of bits that the codec
>>> respects, whereas the sample bits is the number of bits in the sample.
>>> If sample bits - msbits != 0, then that is the number of bits ignored.
>>> Mark basically described it as "feel free to send me more than 24 bits
>>> but I'm ignoring the LSBs".
>>>
>>> If the msbits is limited to 24, then userspace gets to know about that
>>> limitation when negotiating with ALSA if it is a constraint. If it
>>> isn't a constraint, userspace has no way to know.
>>>
>>>> Actually there is very little difference between the case of playing
>>>> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
>>>> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
>>>> ignored like in HDMI audio case.
>>>
>>> You mean 8 LSB throughout the above two paragraphs.
>>>
>>>>> Digging a bit deeper, ASoC passes a private copy of the params to each
>>>>> codec - a copy is made, then fixups for TDM slots are applied, followed
>>>>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
>>>>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
>>>>> back the rate, channels and physical (memory) width and stores them
>>>>> in the codec's DAI structure. The msbits are not read.
>>>>>
>>>>> hdmi-codec also seems to do nothing with the msbits parameter other
>>>>> than the above code.
>>>>>
>>>>> So, all in all, it seems that the above code limiting msbits is
>>>>> redundant - nothing will read this modified value. Can we kill it?
>>>>>
>>>>
>>>> It certainly looks that way, so yes. In any case struct
>>>> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
>>>
>>> This sets a constraint to limit msbits to 24 for any sample width
>>> in core ASoC code, doing exactly what you said "certainly not" to
>>> above.
>>>
>>> It also means that the quoted code in hdmi-codec's hw_params() is
>>> redundant for another reason.
>>>
>>> See the description of:
>>> snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
>>> soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
>>>
>>> Can you please clarify, because there seems to be some confusion.
>>>
>>
>> If I understand right, the only confusion here is around the term
>> "constraint". I did not follow that you meant in strictly alsa-term. Yes
>> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
>> does), because HDMI interface can not deliver more than 24 bits per
>> sample, and any extra LSB bit will be ignored.
>
> Right, so we can remove the above code then?
>
Yes, you correctly pointed out that this piece of code does nothing and
is conceptually wrong:
if (params_width(params) > 24)
params->msbits = 24;
Yes. It can be removed.
--
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
^ permalink raw reply [flat|nested] 17+ messages in thread* Re: hdmi-codec: modifying params in hw_params() callback
@ 2019-02-28 14:41 ` Jyri Sarha
0 siblings, 0 replies; 17+ messages in thread
From: Jyri Sarha @ 2019-02-28 14:41 UTC (permalink / raw)
To: Russell King - ARM Linux admin
Cc: alsa-devel, Mark Brown, Liam Girdwood, linux-arm-kernel,
linux-kernel
On 28/02/2019 16:39, Russell King - ARM Linux admin wrote:
> On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
>> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
>>> On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
>>>> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
>>>>> Hi all,
>>>>>
>>>>> While looking at hdmi-codec issues, I spotted this code:
>>>>>
>>>>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
>>>>> struct snd_pcm_hw_params *params,
>>>>> struct snd_soc_dai *dai)
>>>>> {
>>>>> ...
>>>>> if (params_width(params) > 24)
>>>>> params->msbits = 24;
>>>>>
>>>>> params->msbits is a parameter that is negotiated and refined by
>>>>> libasound, and is part of the ALSA constraint system. The "Writing an
>>>>> ALSA driver" documentation says about the hw_params callback:
>>>>>
>>>>> "This is called when the hardware parameter (``hw_params``) is set up
>>>>> by the application, that is, once when the buffer size, the period
>>>>> size, the format, etc. are defined for the pcm substream."
>>>>>
>>>>> which suggests we should only be reading the parameters, not writing
>>>>> to them.
>>>>>
>>>>> What's more is that the msbits is a parameter that is refined with
>>>>> userspace, so surely the above should be a declared constraint?
>>>>>
>>>>
>>>> Certainly not a constraint. While HDMI can only pass up to 24-bit per
>>>> sample audio the most (or at least the two I have worked with) encoders
>>>> can take 32-bit wide (and probably wider) samples trough i2s. So the
>>>> system can play 32-bit samples, just the 4 LSB is ignored.
>>>
>>> You have not explained why "not a constraint". From the explanation
>>> that Mark gave me on IRC, msbits is the number of bits that the codec
>>> respects, whereas the sample bits is the number of bits in the sample.
>>> If sample bits - msbits != 0, then that is the number of bits ignored.
>>> Mark basically described it as "feel free to send me more than 24 bits
>>> but I'm ignoring the LSBs".
>>>
>>> If the msbits is limited to 24, then userspace gets to know about that
>>> limitation when negotiating with ALSA if it is a constraint. If it
>>> isn't a constraint, userspace has no way to know.
>>>
>>>> Actually there is very little difference between the case of playing
>>>> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
>>>> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
>>>> ignored like in HDMI audio case.
>>>
>>> You mean 8 LSB throughout the above two paragraphs.
>>>
>>>>> Digging a bit deeper, ASoC passes a private copy of the params to each
>>>>> codec - a copy is made, then fixups for TDM slots are applied, followed
>>>>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
>>>>> codec driver's hw_params() callback is made. Afterwards, ASoC reads
>>>>> back the rate, channels and physical (memory) width and stores them
>>>>> in the codec's DAI structure. The msbits are not read.
>>>>>
>>>>> hdmi-codec also seems to do nothing with the msbits parameter other
>>>>> than the above code.
>>>>>
>>>>> So, all in all, it seems that the above code limiting msbits is
>>>>> redundant - nothing will read this modified value. Can we kill it?
>>>>>
>>>>
>>>> It certainly looks that way, so yes. In any case struct
>>>> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
>>>
>>> This sets a constraint to limit msbits to 24 for any sample width
>>> in core ASoC code, doing exactly what you said "certainly not" to
>>> above.
>>>
>>> It also means that the quoted code in hdmi-codec's hw_params() is
>>> redundant for another reason.
>>>
>>> See the description of:
>>> snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
>>> soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
>>>
>>> Can you please clarify, because there seems to be some confusion.
>>>
>>
>> If I understand right, the only confusion here is around the term
>> "constraint". I did not follow that you meant in strictly alsa-term. Yes
>> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
>> does), because HDMI interface can not deliver more than 24 bits per
>> sample, and any extra LSB bit will be ignored.
>
> Right, so we can remove the above code then?
>
Yes, you correctly pointed out that this piece of code does nothing and
is conceptually wrong:
if (params_width(params) > 24)
params->msbits = 24;
Yes. It can be removed.
--
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 17+ messages in thread