Re: [PATCH net-next] net: stmmac: remove superfluous wmb() memory barriers

[Pavel Machek <pavel@ucw.cz>]

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On Fri 2018-03-09 10:26:11, Jose Abreu wrote:
> Hi Niklas,
> 
> On 08-03-2018 10:30, Niklas Cassel wrote:
> > These wmb() memory barriers are performed after the last descriptor write,
> > and they are followed by enable_dma_transmission()/set_tx_tail_ptr(),
> > i.e. a writel() to MMIO register space.
> > Since writel() itself performs the equivalent of a wmb() 
> 
> Sorry but I know at least two architectures which don't do a
> wmb() upon an writel [1] [2]. This can be critical if if we are
> accessing the device through some slow or filled bus which will
> delay accesses to the device IO. Notice that writel and then
> readl to the same address will force CPU to wait for writel
> completion before readl, but in this case we are using DMA and
> then writel so I think a wmb() before the writel is a safe measure.

This also matches documentation, as I tried to point out.

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