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

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Message-Id: <28501fbf-3508-4afe-804d-fa5a3ccd4291@www.fastmail.com>
Date:   Wed, 14 Sep 2022 15:17:17 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Neil Armstrong" <narmstrong@...libre.com>,
        "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 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.

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

     Arnd