Re: [PATCH 2/2] phy: qcom: qmp-pcie: Add PHY register retention support

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 29.01.2025 3:19 PM, neil.armstrong@linaro.org wrote:
> On 29/01/2025 14:55, Konrad Dybcio wrote:
>> On 29.01.2025 2:41 PM, neil.armstrong@linaro.org wrote:
>>> On 29/01/2025 12:29, Konrad Dybcio wrote:
>>>> On 29.01.2025 9:29 AM, neil.armstrong@linaro.org wrote:
>>>>> On 25/01/2025 14:10, Konrad Dybcio wrote:
>>>>>> On 24.01.2025 8:08 AM, Manivannan Sadhasivam wrote:
>>>>>>> + Mayank (with whom I discussed this topic internally)
>>>>>>>
>>>>>>> On Fri, Jan 24, 2025 at 02:22:01PM +0800, Qiang Yu wrote:
>>>>>>>>
>>>>>>>> On 1/22/2025 5:43 PM, Dmitry Baryshkov wrote:
>>>>>>>>> On Wed, Jan 22, 2025 at 03:17:39PM +0800, Wenbin Yao (Consultant) wrote:
>>>>>>>>>> On 1/21/2025 6:36 PM, Dmitry Baryshkov wrote:
>>>>>>>>>>> On Tue, 21 Jan 2025 at 11:43, Wenbin Yao <quic_wenbyao@quicinc.com> wrote:
>>>>>>>>>>>> From: Qiang Yu <quic_qianyu@quicinc.com>
>>>>>>>>>>>>
>>>>>>>>>>>> Currently, BCR reset and PHY register setting are mandatory for every port
>>>>>>>>>>>> before link training. However, some QCOM PCIe PHYs support no_csr reset.
>>>>>>>>>>>> Different than BCR reset that is used to reset entire PHY including
>>>>>>>>>>>> hardware and register, once no_csr reset is toggled, only PHY hardware will
>>>>>>>>>>>> be reset but PHY registers will be retained,
>>>>>>>>>>> I'm sorry, I can't parse this.
>>>>>>>>>> The difference between no_csr reset and bcr reset is that no_csr reset
>>>>>>>>>> doesn't reset the phy registers. If a phy is enabled in UEFI, its registers
>>>>>>>>>> are programed. After Linux boot up, the registers will not be reset but
>>>>>>>>>> keep the value programmed by UEFI if we only do no_csr reset, so we can
>>>>>>>>>> skip phy setting.
>>>>>>>>> Please fix capitalization of the abbreviations (PHY, BCR) and add
>>>>>>>>> similar text to the commit message.
>>>>>>>>>
>>>>>>>>>>>> which means PHY setting can
>>>>>>>>>>>> be skipped during PHY init if PCIe link was enabled in booltloader and only
>>>>>>>>>>>> no_csr is toggled after that.
>>>>>>>>>>>>
>>>>>>>>>>>> Hence, determine whether the PHY has been enabled in bootloader by
>>>>>>>>>>>> verifying QPHY_START_CTRL register. If it is programmed and no_csr reset is
>>>>>>>>>>>> present, skip BCR reset and PHY register setting, so that PCIe link can be
>>>>>>>>>>>> established with no_csr reset only.
>>>>>>>>>>> This doesn't tell us why we want to do so. The general rule is not to
>>>>>>>>>>> depend on the bootloaders at all. The reason is pretty simple: it is
>>>>>>>>>>> hard to update bootloaders, while it is relatively easy to update the
>>>>>>>>>>> kernel. If the hardware team issues any kind of changes to the
>>>>>>>>>>> programming tables, the kernel will get them earlier than the
>>>>>>>>>>> bootloader.
>>>>>>
>>>>>> We're assuming that if a product has shipped, the sequences used to power up
>>>>>> the PHY in the bootloader (e.g. for NVMe) are already good.
>>>>>>
>>>>>> If some tragedy happens and an erratum is needed, we can always introduce a
>>>>>> small override with the existing driver infrastructure (i.e. adding a new
>>>>>> entry with a couple registers worth of programming sequence, leaving the other
>>>>>> values in tact)
>>>>>
>>>>> Assuming Linux will be always ran directly after the bootloader is a wild assumption.
>>>>
>>>> Situations like
>>>>
>>>> [normal boot chain] -> [... (resets the PHY and doesn't reprogram it)] -> Linux
>>>>
>>>> are both so unlikely and so intentional-by-the-user that it doesn't seem
>>>> worth considering really.
>>>
>>> In embedded/mobile/edge world, definitely, in compute/PC-like market, not really.
>>>
>>> You'll have people add some custom bootloaders, hypervisors, who knows what...
>>
>> I see, however you actually have to intentionally assert the non-NO_CSR PHY
>> reset from said custom bootloaders, hypervisors and whoknowswhats for the
>> programmed sequence to be erased. So I have no idea what the issue is here.
> 
> I won't argue further, but you know as I do that relying on the bootloader state
> is a dangerous game, and we already rely a lot for dsp stuff and we have
> a lot lot of issue related to the UEFI implementation already on production
> devices.
> 
> I'm not against the nocsr stuff, which can be a big win for boot time, but
> honestly not adding a few registers in table seems risky enough, and we should
> probably delay this experiment until we are sure the nocsr stuff works fine.

I tested a range of mobile/compute platforms and only the latter kept
the PCIe PHYs initialized after dropping to the OS. No adverse effects
that I can tell.

Konrad