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    SubjectRe: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus block read length
    On Sun, Jun 14, 2020 at 02:02:54PM -0700, Sultan Alsawaf wrote:
    > From: Sultan Alsawaf <sultan@kerneltoast.com>
    >
    > SMBus block reads can be broken because the read function will just skip
    > over bytes it doesn't like until reaching a byte that conforms to the
    > length restrictions for block reads. This is problematic when it isn't
    > known if the incoming payload is indeed a conforming block read.
    >
    > According to the SMBus specification, block reads will only send the
    > payload length in the first byte, so we can fix this by only considering
    > the first byte in a sequence for block read length purposes.

    I'm wondering if this overlaps with [1]. AFAIU that one is also makes sure that
    the length is not a garbage.

    [1]: https://lore.kernel.org/linux-i2c/20200613104109.2989-1-mans@mansr.com/T/#u

    > Fixes: c3ae106050b9 ("i2c: designware: Implement support for SMBus block read and write")
    > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
    > ---
    > drivers/i2c/busses/i2c-designware-master.c | 10 +++++-----
    > 1 file changed, 5 insertions(+), 5 deletions(-)
    >
    > diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
    > index d6425ad6e6a3..16d38b8fc19a 100644
    > --- a/drivers/i2c/busses/i2c-designware-master.c
    > +++ b/drivers/i2c/busses/i2c-designware-master.c
    > @@ -398,7 +398,6 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
    > len += (flags & I2C_CLIENT_PEC) ? 2 : 1;
    > dev->tx_buf_len = len - min_t(u8, len, dev->rx_outstanding);
    > msgs[dev->msg_read_idx].len = len;
    > - msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
    >
    > return len;
    > }
    > @@ -430,10 +429,11 @@ i2c_dw_read(struct dw_i2c_dev *dev)
    > u32 flags = msgs[dev->msg_read_idx].flags;
    >
    > regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
    > - /* Ensure length byte is a valid value */
    > - if (flags & I2C_M_RECV_LEN &&
    > - tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0) {
    > - len = i2c_dw_recv_len(dev, tmp);
    > + if (flags & I2C_M_RECV_LEN) {
    > + /* Ensure length byte is a valid value */
    > + if (tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0)
    > + len = i2c_dw_recv_len(dev, tmp);
    > + msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
    > }
    > *buf++ = tmp;
    > dev->rx_outstanding--;
    > --
    > 2.27.0
    >

    --
    With Best Regards,
    Andy Shevchenko


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