netdev - Re: [PATCH net-next v2 1/6] qed: Add doorbell overflow recovery mechanism

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Message-Id: <20181022.200559.1672641132565115543.davem@davemloft.net>
Date:   Mon, 22 Oct 2018 20:05:59 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     Ariel.Elior@...ium.com
Cc:     netdev@...r.kernel.org, Michal.Kalderon@...ium.com,
        Tomer.Tayar@...ium.com
Subject: Re: [PATCH net-next v2 1/6] qed: Add doorbell overflow recovery
 mechanism

From: Ariel Elior <Ariel.Elior@...ium.com>
Date: Mon, 22 Oct 2018 19:40:40 +0300

>  
> +#ifndef writeq
> +#define writeq writeq
> +static inline void writeq(u64 val, void __iomem *reg)
> +{
> +	writel(val & 0xffffffff, reg);
> +	writel(val >> 32, reg + 0x4UL);
> +}
> +#endif

Please use the appropriate generic header file to achieve this, do not
reimplement it.

See include/linux/io-64-nonatomic*.h and think very carefully about
which one is appropriate.

Specifically, if a register read has side effects but only if you read
the lower or upper half you want to make sure that you use the
implementation that reads the half the doesn't trigger the side
effects first.  This way the whole 64-bit value can be sampled before
status bits clear, or whatever.

Likewise for which half of a register, when written, commits the
results.

If both halfs trigger the side effect or the commit of the write, you
cannot enable your driver on 32-bit architectures.

So this is not such a simple fix where you just hack the build to pass,
you have to really think hard about what the code does, how the hardware
works, and if this can even work properly on 32-bit platforms.

Thank you.