Re: [PATCHv14 2/9] coresight: etm4x: Use asm-generic IO memory barriers

[Sai Prakash Ranjan]

On 5/6/2022 2:51 PM, Suzuki K Poulose wrote:
> On 06/05/2022 04:02, Sai Prakash Ranjan wrote:
> > Hi Suzuki,
> >
> > On 5/6/2022 5:14 AM, Suzuki K Poulose wrote:
> > > Hi,
> > >
> > > On 04/05/2022 12:28, Sai Prakash Ranjan wrote:
> > > > Per discussion in [1], it was decided to move to using architecture
> > > > independent/asm-generic IO memory barriers to have just one set of
> > > > them and deprecate use of arm64 specific IO memory barriers in driver
> > > > code. So replace current usage of __io_rmb()/__iowmb() in drivers to
> > > > __io_ar()/__io_bw().
> > > >
> > > > [1] https://lore.kernel.org/lkml/CAK8P3a0L2tLeF1Q0+0ijUxhGNaw+Z0fyPC1oW6_ELQfn0=i4iw@xxxxxxxxxxxxxx/
> > >
> > > Looking at the dis-assembly it looks like in effect they are slightly
> > > different for arm64.
> > >
> > > i.e., before this patch we had
> > >
> > > "dmb osh{ld/st}"
> > >
> > > and after the patch we have :
> > >
> > > "dsb {ld/st}"
> > >
> > > Is this really what we want ? I don't think this is desirable.
> > >
> > > Suzuki
> >
> > No, this is not supposed to happen and I do not see how it could happen.
> > __io_ar() is defined as __iormb() and __io_bw() is __iowmb().
> >
> > I checked the disassembly in both case with MMIO trace off/on with __etm4_cpu_save()
> > as below and saw the same number of "dmb"s.
> >
> > aarch64-linux-gnu-gdb -batch -ex "disassemble/rs __etm4_cpu_save" vmlinux-without-mmio
> > aarch64-linux-gnu-gdb -batch -ex "disassemble/rs __etm4_cpu_save" vmlinux-with-mmio
> >
> > Can you tell me how are you validating if I am missing something?
>
> Apologies, I was missing the patch in this series, which adds
> the arm64 definition for __io_ar/__io_bw. (hint: Please Cc
> the affected subsystem in the Cover letter for the series, so
> we know what the intention of the changes are).

Sure, I will keep this in mind, apologies for the confusion.

> With the patch1, yes this makes sense to me. Otherwise, __io_ar()
> is default to rmb() which implies dsb ld.
>
> With that said,
>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

Thanks,
Sai