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

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Message-ID: <f3727b53-8da3-d4f6-575b-108a8d6898e5@quicinc.com>
Date: Mon, 19 May 2025 15:04:27 -0700
From: Wesley Cheng <quic_wcheng@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Vinod Koul
	<vkoul@...nel.org>
CC: Melody Olvera <melody.olvera@....qualcomm.com>,
        Kishon Vijay Abraham I
	<kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski
	<krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "Bjorn
 Andersson" <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-phy@...ts.infradead.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 06/10] phy: qcom: Add M31 based eUSB2 PHY driver

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.

Thanks
Wesley Cheng