From: Colin Foster colin.foster@in-advantage.com
[ Upstream commit f0484d2f80a72022b7fac72bcb406392900ef1eb ]
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@in-advantage.com Signed-off-by: Lee Jones lee@kernel.org Link: https://lore.kernel.org/r/20230322141130.2531256-1-colin.foster@in-advantage... Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/mfd/ocelot-spi.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/drivers/mfd/ocelot-spi.c b/drivers/mfd/ocelot-spi.c index 2ecd271de2fb9..85021f94e5874 100644 --- a/drivers/mfd/ocelot-spi.c +++ b/drivers/mfd/ocelot-spi.c @@ -130,6 +130,7 @@ static const struct regmap_config ocelot_spi_regmap_config = {
.write_flag_mask = 0x80,
+ .use_single_read = true, .use_single_write = true, .can_multi_write = false,