Re: [PATCH v4 06/10] phy: qcom: Add M31 based eUSB2 PHY driver

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

On 5/20/25 12:04 AM, Wesley Cheng wrote:
> Hi Konrad,
> 
> On 5/17/2025 11:28 AM, Konrad Dybcio wrote:
>> On 5/14/25 8:24 PM, Wesley Cheng wrote:
>>> Hi Vinod,
>>>
>>> On 5/14/2025 1:33 AM, Vinod Koul wrote:
>>>> On 16-04-25, 15:45, Wesley Cheng wrote:
>>>>> Hi Vinod,
>>>>>
>>>>> On 4/10/2025 4:53 AM, Vinod Koul wrote:
>>>>>> On 09-04-25, 10:48, Melody Olvera wrote:
>>>>>>
>>>>>>> +static int m31eusb2_phy_write_readback(void __iomem *base, u32 offset,
>>>>>>> +                    const u32 mask, u32 val)
>>>>>>> +{
>>>>>>> +    u32 write_val;
>>>>>>> +    u32 tmp;
>>>>>>> +
>>>>>>> +    tmp = readl_relaxed(base + offset);
>>>>>>> +    tmp &= ~mask;
>>>>>>> +    write_val = tmp | val;
>>>>>>> +
>>>>>>> +    writel_relaxed(write_val, base + offset);
>>>>>>> +
>>>>>>> +    tmp = readl_relaxed(base + offset);
>>>>>>
>>>>>> Why are you using _relaxed version here?
>>>>>>
>>>>>
>>>>> No particular reason.  I think someone pointed this out previously, and I
>>>>> was open to use the non-relaxed variants, but I assume using the relaxed vs
>>>>> non-relaxed apis comes down to preference in this case.
>>>>
>>>> Nope you cant! There _needs_ to be a specific reasons!
>>>> When you are doing read, modify, write, it is very important to know the
>>>> right version to use...
>>>>
>>>
>>> I mean, its a write readback, which ensures the bus transaction is complete
>>> based on [1], hence why **in this situation** it is up to preference.
>>>
>>> Otherwise, w/o the readback then we'd need to ensure writes are made
>>> depending on the required sequencing (in spots where the sequence is
>>> strictly defined), and that can be enforced using barriers.  If you feel
>>> like using the non-relaxed variant is preferred let me know.  I can replace
>>> it and remove the readback.
>>
>> Readback is stronger on arm64, as otherwise the writes may be buffered and
>> not observable at the other endpoint even though the instruction has been
>> issued, even if a barrier has been issued
>>
>> Some resources:
>>
>> https://youtu.be/i6DayghhA8Q
>> https://lore.kernel.org/linux-arm-msm/20240618153419.GC2354@willie-the-truck/
>> https://developer.arm.com/documentation/ddi0487/latest sec B2.6.9
>>
>> There's been a real bug observed (pun not intended):
>> Commit 2f8cf2c3f3e3 ("clk: qcom: reset: Ensure write completion on reset de/assertion")
>>
> 
> Thanks for sharing.  Useful info...The way I interpret it, even between relaxed and non-relaxed variants, a readback is always desired.

Yes, and IIRC it didn't exactly matter which of the parameters was set
first, so we can use relaxed

Konrad