51 lines
1.7 KiB
Diff
51 lines
1.7 KiB
Diff
From 72742fb9d06e6c931738de020bc6bbd98c6829f7 Mon Sep 17 00:00:00 2001
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From: Phil Elwell <phil@raspberrypi.com>
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Date: Thu, 28 Jan 2021 11:30:04 +0000
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Subject: [PATCH] spi: bcm2835: Workaround/fix for zero-length
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transfers
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A relatively recent commit ([1]) contained optimisation for the PIO
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SPI FIFO-filling functions. The commit message includes the phrase
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"[t]he blind and counted loops are always called with nonzero count".
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This is technically true, but it is still possible for count to become
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zero before the loop is entered - if tfr->len is zero. Moving the loop
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exit condition to the end of the loop saves a few cycles, but results
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in a near-infinite loop should the revised count be zero on entry.
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Strangely, zero-lengthed transfers aren't filtered by the SPI framework
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and, even more strangely, the Python3 spidev library is triggering them
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for no obvious reason.
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Avoid the problem completely by bailing out of the main transfer
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function early if trf->len is zero, although there may be a case for
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moving the mitigation into the framework.
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See: https://github.com/raspberrypi/linux/issues/4100
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Signed-off-by: Phil Elwell <phil@raspberrypi.com>
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[1] 26751de25d25 ("spi: bcm2835: Micro-optimise FIFO loops")
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---
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drivers/spi/spi-bcm2835.c | 10 ++++++++++
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1 file changed, 10 insertions(+)
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--- a/drivers/spi/spi-bcm2835.c
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+++ b/drivers/spi/spi-bcm2835.c
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@@ -1055,6 +1055,16 @@ static int bcm2835_spi_transfer_one(stru
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unsigned long hz_per_byte, byte_limit;
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u32 cs = slv->prepare_cs;
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+ if (unlikely(!tfr->len)) {
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+ static int warned;
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+
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+ if (!warned)
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+ dev_warn(&spi->dev,
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+ "zero-length SPI transfer ignored\n");
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+ warned = 1;
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+ return 0;
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+ }
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+
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/* set clock */
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spi_hz = tfr->speed_hz;
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