Re: [PATCH v9 27/28] media: iris: enable video driver probe of SM8250 SoC

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

Em Wed, 8 Jan 2025 16:42:03 +0530
Dikshita Agarwal <quic_dikshita@quicinc.com> escreveu:

> On 1/8/2025 4:13 PM, Hans Verkuil wrote:
> > On 1/8/25 11:21, Dikshita Agarwal wrote:  
> >>
> >>
> >> On 1/8/2025 2:25 PM, Hans Verkuil wrote:  
> >>> On 08/01/2025 09:51, Dikshita Agarwal wrote:  
> >>>>
> >>>>
> >>>> On 1/8/2025 1:17 PM, Hans Verkuil wrote:  
> >>>>> On 08/01/2025 08:43, Dikshita Agarwal wrote:  
> >>>>>>
> >>>>>>
> >>>>>> On 1/7/2025 7:27 PM, Nicolas Dufresne wrote:  
> >>>>>>> Le lundi 23 décembre 2024 à 16:21 +0530, Dikshita Agarwal a écrit :  
> >>>>>>>>
> >>>>>>>> On 12/23/2024 4:00 PM, Mauro Carvalho Chehab wrote:  
> >>>>>>>>> Em Thu, 12 Dec 2024 17:21:49 +0530
> >>>>>>>>> Dikshita Agarwal <quic_dikshita@quicinc.com> escreveu:
> >>>>>>>>>  
> >>>>>>>>>> +	.dma_mask = GENMASK(31, 29) - 1,  
> >>>>>>>>>
> >>>>>>>>> Setting a mask to GENMASK() - 1 sounds weird. Is it really what you want?
> >>>>>>>>> I so, why?
> >>>>>>>>>  
> >>>>>>>> Hi Mauro,
> >>>>>>>>
> >>>>>>>> the value of this dma mask should be 0xe0000000 -1.
> >>>>>>>>
> >>>>>>>> The background for the same is, 0xe0000000 onward memory space is allocated
> >>>>>>>> for IO register space so we are restricting the driver buffer allocations
> >>>>>>>> to 0xe0000000 - 1.
> >>>>>>>>
> >>>>>>>> Based on the comments received in the past, we are using GENMASK to
> >>>>>>>> generate 0xe0000000.
> >>>>>>>>
> >>>>>>>> Does this answer your query or I missed something?  
> >>>>>>>
> >>>>>>> I'm not sure it will do what you want. (0xe0000000 -1) matches ~BIT(29). Perhaps
> >>>>>>> you wanted to use ~0xe0000000. 
> >>>>>>>  
> >>>>>> value of dma mask is coming as expected with GENMASK(31, 29) - 1
> >>>>>>
> >>>>>> qcom-iris aa00000.video-codec: dma_mask DFFFFFFF (0xe0000000 -1)  
> >>>>>
> >>>>> Isn't this just the equivalent of GENMASK(28, 0)? Can't you use that?  
> >>>
> >>> Too early in the morning, this suggestion was clearly wrong.
> >>>  
> >>>>>
> >>>>> It's much easier to understand than GENMASK()-1.  
> >>>>
> >>>> Sure, I can use either ~GENMASK(29, 29) or ~BIT(29),  
> >>>
> >>> ~BIT(29).
> >>>
> >>> It's really weird to just disable a single bit, so I think some comments
> >>> explaining why this mask is needed would be good (if there aren't comments
> >>> already).
> >>>  
> >> I tested this some more, and seems ~BIT(29) doesn't work, as its still
> >> conflicting with the register space.  
> > 
> > Odd, perhaps a 64 vs 32 bit issue?
> >   
> >> Correct value would be GENMASK(31,30) + GENMASK(28,0) to set the exact bits
> >> to get the desired value i.e 0xe0000000 -1  
> > Honestly, in this case I would prefer to just go with the actual hex value
> > 0xdfffffff together with an explanatory comment.
> >   
> We moved to GENMASK way to address comment on previous version, but sure
> can directly use 0xdfffffff with a comment.

If I understood it right, bits 0-31 can be used, but the hardware has some
issue using bit 29 at the mask. Could you please comment why it can't be
used?

Btw, as this is a mask, IMO the better would be to document that all bits
except for BIT(29) can be used with something like:

	/* Bit 29 can't be used because ... */
	 .dma_mask = GENMASK(31, 0) - BIT(29)

Thanks,
Mauro