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

[Wesley Cheng]

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