linux-kernel - Re: [PATCH] spi: meson-spicc: add support for DMA

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Date:   Wed, 14 Sep 2022 15:43:39 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...nel.org>
Cc:     linux-spi@...r.kernel.org, linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: meson-spicc: add support for DMA

On 14/09/2022 15:17, Arnd Bergmann wrote:
> On Wed, Sep 14, 2022, at 2:35 PM, Neil Armstrong wrote:
>> On 14/09/2022 13:19, Arnd Bergmann wrote:
>>> On Tue, Sep 13, 2022, at 4:03 PM, Neil Armstrong wrote:
>>>
>>>> + /* Sometimes, TC gets triggered while the RX fifo isn't fully flushed *
>>>> + if (spicc->using_dma) {
>>>> +          unsigned int rxfifo_count = FIELD_GET(SPICC_RXCNT_MASK,
>>>> +                       readl_relaxed(spicc->base + SPICC_TESTREG));
>>>
>>> Same here in the interrupt controller, I don't see anything enforcing
>>> the DMA to actually complete before the readl_relaxed().
>>
>> I don't see the relathionship between a register relaxed read and the
>> DMA not finishing
>> writing the data in uncached memory, for me it's 2 unrelated things.
> 
> The race is between the readl_relaxed() and a subsequent access
> to the data that is being transferred. On Arm processors you
> need a "dmb(oshld)" instruction to ensure that the CPU cannot
> prefetch data from the DMA buffer while it is waiting for the
> MMIO to complete.
> 
> The __io_ar() in readl() exists specifically there for this race, and
> this is the reason that readl_relaxed() exists for drivers that
> do not do any DMA.

Acked, thx for for expliciting, will do the fix.

> 
> Note that this prefetching can happen for uncached memory, but
> spe-meson-spicc uses cached memory.
> 
>       Arnd