[PATCH] i2c: omap: ensure writes to dev->buf_len are ordered
From: paul@pwsan.com (Paul Walmsley)
Date: 2012-10-26 23:01:11
Also in:
linux-i2c, linux-omap
Hi Felipe just two quick comments On Thu, 25 Oct 2012, Felipe Balbi wrote:
quoted hunk ↗ jump to hunk
if we allow compiler reorder our writes, we could fall into a situation where dev->buf_len is reset for no apparent reason. This bug was found with a simple script which would transfer data to an i2c client from 1 to 1024 bytes (a simple for loop), when we got to transfer sizes bigger than the fifo size, dev->buf_len was reset to zero before we had an oportunity to handle XDR Interrupt. Because dev->buf_len was zero, we entered omap_i2c_transmit_data() to transfer zero bytes, which would mean we would just silently exit omap_i2c_transmit_data() without actually writing anything to DATA register. That would cause XDR IRQ to trigger forever and we would never transfer the remaining bytes. After adding the memory barrier, we also drop resetting dev->buf_len to zero in omap_i2c_xfer_msg() because both omap_i2c_transmit_data() and omap_i2c_receive_data() will act until dev->buf_len reaches zero, rendering the other write in omap_i2c_xfer_msg() redundant. This patch has been tested with pandaboard for a few iterations of the script mentioned above. Signed-off-by: Felipe Balbi <redacted> --- This bug has been there forever, but it's quite annoying. I think it deserves being pushed upstream during this -rc cycle, but if Wolfram decides to wait until v3.8, I don't mind. drivers/i2c/busses/i2c-omap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c index db31eae..1ec4e6e 100644 --- a/drivers/i2c/busses/i2c-omap.c +++ b/drivers/i2c/busses/i2c-omap.c@@ -521,6 +521,7 @@ static int omap_i2c_xfer_msg(struct i2c_adapter *adap, /* REVISIT: Could the STB bit of I2C_CON be used with probing? */ dev->buf = msg->buf; dev->buf_len = msg->len; + wmb(); omap_i2c_write_reg(dev, OMAP_I2C_CNT_REG, dev->buf_len);
Would suggest moving the wmb() immediately before the point at which the
interrupt can occur. Looks to me that's when the OMAP_I2C_CON_REG write
occurs.
Also would suggest adding a comment to clarify what the wmb() is intended
to do. Maybe something like 'Prevent the compiler from moving earlier
changes to dev->buf and dev->buf_len after the write to CON_REG. This
write enables interrupts and those variables are used in the interrupt
handler'.
checkpatch is supposed to flag uncommented barriers, but maybe that's only
with --strict.
# check for memory barriers without a comment.
if ($line =~ /\b(mb|rmb|wmb|read_barrier_depends|smp_mb|smp_rmb|smp_wmb|smp_read_barrier_depends)\(/) {
if (!ctx_has_comment($first_line, $linenr)) {
CHK("MEMORY_BARRIER",
"memory barrier without comment\n" .
$herecurr);
}
}
- Paul