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

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

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.

Konrad

> 
>>
>> If whatever sits in the middle *must* hard-reset the phy, it can save the
>> register state beforehand and restore them after the reset
>>
>>> Yes, we should make use the noscr if the PHY is always programmed, but we should be
>>> always able to reprogram the PHY entirely to recover a faulty programmation.
>>
>> We aren't considering any possibility of faulty programming - it's either
>> programmed, or not. And if the values configured by the bootloader are wrong,
>> the device's firmware is considered faulty.
>>
>> Most devices probably follow the exact same magic values as our reference
>> boards (though these values relate to analog characteristics, so perhaps not
>> *all* of them, which is another argument for keeping the BL state) and these
>> are extensively tested internally before any production devices make it out
>> the door. Any updates deep into the product life are most likely just "nice
>> to have"s and not anything critical, and as I've mentioned, we can still have
>> overrides with the current logic inside this driver.
>>
>> Konrad
>