Subject: Re: [PATCH net-next 2/2] mlx5/core: deduplicate {mlx5_,}eq_update_ci()

From: Saeed Mahameed <[email-masked]>
Date: Wed, 6 Nov 2024 21:14:27 -0800

On 06 Nov 20:45, Caleb Sander wrote:
>On Wed, Nov 6, 2024 at 6:36 PM Saeed Mahameed <[email-masked]> wrote:
>>
>> On 06 Nov 15:44, Caleb Sander wrote:
>> >On Tue, Nov 5, 2024 at 9:44 PM Parav Pandit <[email-masked]> wrote:
>> >>
>> >>
>> >> > From: Caleb Sander <[email-masked]>
>> >> > Sent: Tuesday, November 5, 2024 9:36 PM
>> >> >
>> >> > On Mon, Nov 4, 2024 at 9:22 PM Parav Pandit <[email-masked]> wrote:
>> >> > >
>> >> > >
>> >> > >
>> >> > > > From: Caleb Sander <[email-masked]>
>> >> > > > Sent: Monday, November 4, 2024 3:49 AM
>> >> > > >
>> >> > > > On Sat, Nov 2, 2024 at 8:55 PM Parav Pandit <[email-masked]> wrote:
>> >> > > > >
>> >> > > > >
>> >> > > > >
>> >> > > > > > From: Caleb Sander Mateos <[email-masked]>
>> >> > > > > > Sent: Friday, November 1, 2024 9:17 AM
>> >> > > > > >
>> >> > > > > > The logic of eq_update_ci() is duplicated in mlx5_eq_update_ci().
>> >> > > > > > The only additional work done by mlx5_eq_update_ci() is to
>> >> > > > > > increment
>> >> > > > > > eq->cons_index. Call eq_update_ci() from mlx5_eq_update_ci() to
>> >> > > > > > eq->avoid
>> >> > > > > > the duplication.
>> >> > > > > >
>> >> > > > > > Signed-off-by: Caleb Sander Mateos <[email-masked]>
>> >> > > > > > ---
>> >> > > > > >  drivers/net/ethernet/mellanox/mlx5/core/eq.c | 9 +--------
>> >> > > > > >  1 file changed, 1 insertion(+), 8 deletions(-)
>> >> > > > > >
>> >> > > > > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/eq.c
>> >> > > > > > b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
>> >> > > > > > index 859dcf09b770..078029c81935 100644
>> >> > > > > > --- a/drivers/net/ethernet/mellanox/mlx5/core/eq.c
>> >> > > > > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
>> >> > > > > > @@ -802,19 +802,12 @@ struct mlx5_eqe *mlx5_eq_get_eqe(struct
>> >> > > > > > mlx5_eq *eq, u32 cc)  }  EXPORT_SYMBOL(mlx5_eq_get_eqe);
>> >> > > > > >
>> >> > > > > >  void mlx5_eq_update_ci(struct mlx5_eq *eq, u32 cc, bool arm)  {
>> >> > > > > > -     __be32 __iomem *addr = eq->doorbell + (arm ? 0 : 2);
>> >> > > > > > -     u32 val;
>> >> > > > > > -
>> >> > > > > >       eq->cons_index += cc;
>> >> > > > > > -     val = (eq->cons_index & 0xffffff) | (eq->eqn << 24);
>> >> > > > > > -
>> >> > > > > > -     __raw_writel((__force u32)cpu_to_be32(val), addr);
>> >> > > > > > -     /* We still want ordering, just not swabbing, so add a barrier */
>> >> > > > > > -     wmb();
>> >> > > > > > +     eq_update_ci(eq, arm);
>> >> > > > > Long ago I had similar rework patches to get rid of
>> >> > > > > __raw_writel(), which I never got chance to push,
>> >> > > > >
>> >> > > > > Eq_update_ci() is using full memory barrier.
>> >> > > > > While mlx5_eq_update_ci() is using only write memory barrier.
>> >> > > > >
>> >> > > > > So it is not 100% deduplication by this patch.
>> >> > > > > Please have a pre-patch improving eq_update_ci() to use wmb().
>> >> > > > > Followed by this patch.
>> >> > > >
>> >> > > > Right, patch 1/2 in this series is changing eq_update_ci() to use
>> >> > > > writel() instead of __raw_writel() and avoid the memory barrier:
>> >> > > > https://lore.kernel.org/lkml/20241101034647.51590-1-
>> >> > > > [email-masked]/
>> >> > > This patch has two bugs.
>> >> > > 1. writel() writes the MMIO space in LE order. EQ updates are in BE order.
>> >> > > So this will break on ppc64 BE.
>> >> >
>> >> > Okay, so this should be writel(cpu_to_le32(val), addr)?
>> >> >
>> >> That would break the x86 side because device should receive in BE format regardless of cpu endianness.
>> >> Above code will write in the LE format.
>> >>
>> >> So an API foo_writel() need which does
>> >> a. write memory barrier
>> >> b. write to MMIO space but without endineness conversion.
>> >
>> >Got it, thanks. writel(bswap_32(val, addr)) should work, then? I
>> >suppose it may introduce a second bswap on BE architectures, but
>> >that's probably worth it to avoid the memory barrier.
>> >
>>
>> The existing mb() needs to be changed to wmb(), this will provide a more
>> efficient fence on most architectures.
>>
>> I don't understand why you are still discussing the use of writel(), yes
>> it will work but you are introducing two unconditional swaps per doorbell
>> write.
>
>Well, no memory fence is cheaper still than a wmb(). But it's your
>driver, so if you prefer to use wmb() rather than switch to writel(),
>that's fine. I'll update the patch series.

yest wmb() please.

>As for the bytes swaps in writel(bswap_32(val), addr), it would still
>be 1 on LE architectures, but 2 instead of 0 on BE architectures.
>Certainly a bit inefficient, but probably less overhead than the
>memory barrier currently adds on strongly-ordered architectures.
>

yes we all agree:

mb << writel() < _raw_writel() + wmb () 

>>
>> Just replace the existing mb with wmb() in eq_update_ci()
>>
>> And if you have time to write one extra patch, please reuse eq_update_ci()
>> inside mlx5_eq_update_ci().
>>
>> mlx5_eq_update_ci(eq, cc, arm) {
>>         eq->cons_index += cc;
>>         eq_update_ci(eq, arm);
>> }
>>
>> So we won't have two different implementations of EQ doorbell ringing
>> anymore.
>
>Isn't this what my patch 2 (at the start of this reply chain) already
>does? If you are suggesting something else, please clarify.
>

Sorry i missed it, 

Yes this is perfect, thanks for the patches.
Let's do the wmb() and call it a day :) ..