linux-kernel - Re: [PATCH v1 mfd] mfd: ocelot-spi: fix bulk read

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Date:   Wed, 22 Mar 2023 08:45:32 -0700
From:   Colin Foster <colin.foster@...advantage.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     linux-kernel@...r.kernel.org, Lee Jones <lee@...nel.org>
Subject: Re: [PATCH v1 mfd] mfd: ocelot-spi: fix bulk read

On Wed, Mar 22, 2023 at 05:25:51PM +0200, Vladimir Oltean wrote:
> On Wed, Mar 22, 2023 at 07:11:30AM -0700, Colin Foster wrote:
> > Ocelot chips (VSC7511, VSC7512, VSC7513, VSC7514) don't support bulk read
> > operations over SPI.
> > 
> > Many SPI buses have hardware that can optimize consecutive reads.
> > Essentially an address is written to the chip, and if the SPI controller
> > continues to toggle the clock, subsequent register values are reported.
> > This can lead to significant optimizations, because the time between
> > "address is written to the chip" and "chip starts to report data" can often
> > take a fixed amount of time.
> > 
> > When support for Ocelot chips were added in commit f3e893626abe ("mfd:
> > ocelot: Add support for the vsc7512 chip via spi") it was believed that
> > this optimization was supported. However it is not.
> 
> Details? What about bulk reads is "not supported", and not supported by whom?

The chip itself doesn't support bulk reads. Every register read must be
"Chip Select \" > "Read+Address Command" > "Padding" > "One Register Value" >
"Chip Select /"

Figure 74 of [1] shows "SI Read Timing in Fast Mode", but that is when
the VSC751X is the SPI controller, not a SPI endpoint. i.e. when the
VSC751X is reading _from_ an external flash chip. It also has a blurb
about "After reading address n, the SI boot controller automatically
continues reading address n+1".

Figure 63 shows "Read Sequence for One-Byte Padding" which is actually
done when an external CPU is reading _from_ the VSC751X devices. There
is no suggestion that address n+1 will be returned in this scenario.

[1] https://ww1.microchip.com/downloads/en/DeviceDoc/VMDS-10489.pdf

I can update the commit message as needed.