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

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Message-ID: <20230330123753.GI434339@google.com>
Date:   Thu, 30 Mar 2023 13:37:53 +0100
From:   Lee Jones <lee@...nel.org>
To:     Colin Foster <colin.foster@...advantage.com>
Cc:     linux-kernel@...r.kernel.org,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH v1 mfd] mfd: ocelot-spi: fix bulk read

On Wed, 22 Mar 2023, 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.
>
> Most register transactions with the Ocelot chips are not done in bulk, so
> this bug could go unnoticed. The one scenario where bulk register
> operations _are_ performed is when polling port statistics counters, which
> was added in commit d87b1c08f38a ("net: mscc: ocelot: use bulk reads for
> stats").
>
> Things get slightly more complicated here...
>
> A bug was introduced in commit d4c367650704 ("net: mscc: ocelot: keep
> ocelot_stat_layout by reg address, not offset") that broke the optimization
> of bulk reads. This means that when Ethernet support for the VSC7512 chip
> was added in commit 3d7316ac81ac ("net: dsa: ocelot: add external ocelot
> switch control") things were actually working "as expected".
>
> The bulk read opmtimization was discovered, and fixed in commit
> 6acc72a43eac ("net: mscc: ocelot: fix stats region batching") and the
> timing optimizations for SPI were noticed. A bulk read went from ~14ms to
> ~2ms. But this timing improvement came at the cost of every register
> reading zero due the fact that bulk reads don't work.
>
> The read timings increase back to 13-14ms, but that's a price worth paying
> in order to receive valid data. This is verified in a DSA setup (cpsw-new
> switch tied to port 0 on the VSC7512, after having been running overnight)
>
>      Rx Octets: 16222055 # Counters from CPSW switch
>      Tx Octets: 12034702
>      Net Octets: 28256757
>      p00_rx_octets: 12034702 # Counters from Ocelot switch
>      p00_rx_frames_below_65_octets: 0
>      p00_rx_frames_65_to_127_octets: 88188
>      p00_rx_frames_128_to_255_octets: 13
>      p00_rx_frames_256_to_511_octets: 0
>      p00_rx_frames_512_to_1023_octets: 0
>      p00_rx_frames_over_1526_octets: 3306
>      p00_tx_octets: 16222055
>
> Fixes: f3e893626abe ("mfd: ocelot: Add support for the vsc7512 chip via spi")
> Signed-off-by: Colin Foster <colin.foster@...advantage.com>
> ---
>  drivers/mfd/ocelot-spi.c | 1 +
>  1 file changed, 1 insertion(+)

Applied, thanks

--
Lee Jones [李琼斯]