linux-kernel - Re: [PATCH 3.16 167/294] i2c: ismt: Don't duplicate the receive length for block reads

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Message-ID: <0648c7a9-a683-46f1-55b5-ac9faa194262@adiengineering.com>
Date:   Tue, 7 Nov 2017 11:24:36 -0500
From:   Stephen Douthit <stephend@...engineering.com>
To:     Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     akpm@...ux-foundation.org, Dan Priamo <danp@...engineering.com>,
        Neil Horman <nhorman@...driver.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Pontus Andersson <epontan@...il.com>
Subject: Re: [PATCH 3.16 167/294] i2c: ismt: Don't duplicate the receive
 length for block reads

On 11/06/2017 06:03 PM, Ben Hutchings wrote:
> 3.16.50-rc1 review patch.  If anyone has any objections, please let me know.

Pontus found that this patch trades one bug for another (fixes SMBus
reads, breaks I2C reads) and provided a fix.

You'll also want c6ebcedbab7ca78984959386012a17b21183e1a3 from upstream.

-Steve

> ------------------
> 
> From: Stephen Douthit <stephend@...engineering.com>
> 
> commit b6c159a9cb69c2cf0bf59d4e12c3a2da77e4d994 upstream.
> 
> According to Table 15-14 of the C2000 EDS (Intel doc #510524) the
> rx data pointed to by the descriptor dptr contains the byte count.
> 
> desc->rxbytes reports all bytes read on the wire, including the
> "byte count" byte.  So if a device sends 4 bytes in response to a
> block read, on the wire and in the DMA buffer we see:
> 
> count data1 data2 data3 data4
>   0x04  0xde  0xad  0xbe  0xef
> 
> That's what we want to return in data->block to the next level.
> 
> Instead we were actually prefixing that with desc->rxbytes:
> 
> bad
> count count data1 data2 data3 data4
>   0x05  0x04  0xde  0xad  0xbe  0xef
> 
> This was discovered while developing a BMC solution relying on the
> ipmi_ssif.c driver which was trying to interpret the bogus length
> field as part of the IPMI response.
> 
> Signed-off-by: Stephen Douthit <stephend@...engineering.com>
> Tested-by: Dan Priamo <danp@...engineering.com>
> Acked-by: Neil Horman <nhorman@...driver.com>
> Signed-off-by: Wolfram Sang <wsa@...-dreams.de>
> Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> ---
>   drivers/i2c/busses/i2c-ismt.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/drivers/i2c/busses/i2c-ismt.c
> +++ b/drivers/i2c/busses/i2c-ismt.c
> @@ -344,8 +344,8 @@ static int ismt_process_desc(const struc
>   			break;
>   		case I2C_SMBUS_BLOCK_DATA:
>   		case I2C_SMBUS_I2C_BLOCK_DATA:
> -			memcpy(&data->block[1], dma_buffer, desc->rxbytes);
> -			data->block[0] = desc->rxbytes;
> +			memcpy(data->block, dma_buffer, desc->rxbytes);
> +			data->block[0] = desc->rxbytes - 1;
>   			break;
>   		}
>   		return 0;
>