linux-kernel - RE: [PATCH] locking/memory-barriers.txt: Improve documentation for writel() usage

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Date:   Thu, 15 Sep 2022 14:18:15 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Arnd Bergmann <arnd@...db.de>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "akiyks@...il.com" <akiyks@...il.com>,
        Dan Lustig <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: RE: [PATCH] locking/memory-barriers.txt: Improve documentation for
 writel() usage

> From: Arnd Bergmann <arnd@...db.de>
> Sent: Thursday, September 15, 2022 8:38 AM
> 
> On Thu, Sep 15, 2022, at 7:01 AM, Parav Pandit wrote:
> > The cited commit [1] describes that when using writel(), explcit wmb()
> > is not needed. However, it should have said that dma_wmb() is not
> > needed.
> 
> Are you sure? As I understand it, the dma_wmb() only serializes a set of
> memory accesses, but does not serialized against an MMIO access, which
> depending on the CPU architecture may require a different type of barrier.
> 
> E.g. on arm, writel() uses __iowmb(), which like wmb() is defined as "dsb(x);
> arm_heavy_mb();", while dma_wmb() is a "dmb(oshst)".

You are right, on arm heavy barrier dsb() is needed, while on arm64, dmb(oshst) is sufficient.

So more accurate documentation is to say that 
'when using writel() a prior IO barrier is not needed ...'

How about that?

It started with my cleanup efforts to two drivers [1] and [2] that had difficulty in using writel() on 32-bit system, and it ended up open coding writel() as wmb() + mlx5_write64().

I am cleaning up the repetitive pattern of, 
wmb();
mlx5_write64()

Before I fix drivers, I thought to improve the documentation that I can follow. :)

[1] https://elixir.bootlin.com/linux/latest/source/drivers/infiniband/hw/mlx5/wr.c#L1042
[2] https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h#L226