linux-kernel - Re: [PATCH 2/6] spi: sun4i: restrict transfer length in PIO-mode

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Message-ID: <20180403114006.u7tz2c7lvofsqwtg@flea>
Date:   Tue, 3 Apr 2018 13:40:06 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Sergey Suloev <ssuloev@...altech.com>
Cc:     Mark Brown <broonie@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] spi: sun4i: restrict transfer length in PIO-mode

On Tue, Apr 03, 2018 at 02:08:43PM +0300, Sergey Suloev wrote:
> On 04/03/2018 11:10 AM, Maxime Ripard wrote:
> > On Thu, Mar 29, 2018 at 09:59:03PM +0300, Sergey Suloev wrote:
> > > There is no need to handle 3/4 empty/full interrupts as the maximum
> > > supported transfer length in PIO mode is 64 bytes for sun4i-family
> > > SoCs.
> > That assumes that you'll be able to treat the FIFO full interrupt and
> > drain the FIFO before we have the next byte coming in. This would
> > require a real time system, and we're not in one of them.
>
> AFAIK in SPI protocol we send and receive at the same time.

It depends. The protocol allows it yes, but most devices I've seen can
only operate in half duplex. But it's not really the point.

> As soon as the transfer length is <= FIFO depth then it means that
> at the moment we get TC interrupt all data for this transfer
> sent/received already.
> 
> Is your point here that draining FIFO might be a long operation and we can
> lose next portion of data ?

My point is that, if you get another interrupt(s) right before the
FIFO full interrupt, that interrupt is going to be masked for as long
as it is needed for the previous handler(s) to execute.

If you're having another byte received while the interrupt is masked,
you're losing data.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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