From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-wr0-f193.google.com ([209.85.128.193]:34991 "EHLO mail-wr0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751373AbdCZOYR (ORCPT ); Sun, 26 Mar 2017 10:24:17 -0400 Received: by mail-wr0-f193.google.com with SMTP id p52so2551636wrc.2 for ; Sun, 26 Mar 2017 07:24:16 -0700 (PDT) Subject: Re: [PATCH 2/2] em28xx: simplify ID-reading from Micron sensors To: Mauro Carvalho Chehab References: <20170219182918.4978-1-fschaefer.oss@googlemail.com> <20170219182918.4978-2-fschaefer.oss@googlemail.com> <20170322114606.1feeb960@vento.lan> <5107179d-74fd-3b98-5fc6-ba7051927ae2@googlemail.com> <20170323095612.72216892@vento.lan> <84459d79-eccc-1888-1dad-6935cf85b18a@googlemail.com> <20170324161616.74e5dbc7@vento.lan> Cc: linux-media@vger.kernel.org, mchehab@kernel.org, arnd@arndb.de From: =?UTF-8?Q?Frank_Sch=c3=a4fer?= Message-ID: <2f06c7bb-1951-fdce-8890-470be73c15ff@googlemail.com> Date: Sun, 26 Mar 2017 16:24:17 +0200 MIME-Version: 1.0 In-Reply-To: <20170324161616.74e5dbc7@vento.lan> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-media-owner@vger.kernel.org List-ID: Am 24.03.2017 um 20:16 schrieb Mauro Carvalho Chehab: > Em Thu, 23 Mar 2017 19:03:20 +0100 > Frank Schäfer escreveu: > >> Am 23.03.2017 um 13:56 schrieb Mauro Carvalho Chehab: >>> Em Thu, 23 Mar 2017 13:01:32 +0100 >>> Frank Schäfer escreveu: >>> >>>> Am 22.03.2017 um 15:46 schrieb Mauro Carvalho Chehab: >>>>> Em Sun, 19 Feb 2017 19:29:18 +0100 >>>>> Frank Schäfer escreveu: >>>>> >>>>>> Use i2c_smbus_read_word_data() instead of i2c_master_send() and >>>>>> i2c_master_recv() for reading the ID of Micorn sensors. >>>>>> Bytes need to be swapped afterwards, because i2c_smbus_read_word_data() >>>>>> assumes that the received bytes are little-endian byte order (as specified >>>>>> by smbus), while Micron sensors with 16 bit register width use big endian >>>>>> byte order. >>>>>> >>>>>> Signed-off-by: Frank Schäfer >>>>>> --- >>>>>> drivers/media/usb/em28xx/em28xx-camera.c | 28 ++++------------------------ >>>>>> 1 file changed, 4 insertions(+), 24 deletions(-) >>>>>> >>>>>> diff --git a/drivers/media/usb/em28xx/em28xx-camera.c b/drivers/media/usb/em28xx/em28xx-camera.c >>>>>> index 7b4129ab1cf9..4839479624e7 100644 >>>>>> --- a/drivers/media/usb/em28xx/em28xx-camera.c >>>>>> +++ b/drivers/media/usb/em28xx/em28xx-camera.c >>>>>> @@ -106,8 +106,6 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev) >>>>>> { >>>>>> int ret, i; >>>>>> char *name; >>>>>> - u8 reg; >>>>>> - __be16 id_be; >>>>>> u16 id; >>>>>> >>>>>> struct i2c_client *client = &dev->i2c_client[dev->def_i2c_bus]; >>>>>> @@ -115,10 +113,8 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev) >>>>>> dev->em28xx_sensor = EM28XX_NOSENSOR; >>>>>> for (i = 0; micron_sensor_addrs[i] != I2C_CLIENT_END; i++) { >>>>>> client->addr = micron_sensor_addrs[i]; >>>>>> - /* NOTE: i2c_smbus_read_word_data() doesn't work with BE data */ >>>>>> /* Read chip ID from register 0x00 */ >>>>>> - reg = 0x00; >>>>>> - ret = i2c_master_send(client, ®, 1); >>>>>> + ret = i2c_smbus_read_word_data(client, 0x00); /* assumes LE */ >>>>>> if (ret < 0) { >>>>>> if (ret != -ENXIO) >>>>>> dev_err(&dev->intf->dev, >>>>>> @@ -126,24 +122,9 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev) >>>>>> client->addr << 1, ret); >>>>>> continue; >>>>>> } >>>>>> - ret = i2c_master_recv(client, (u8 *)&id_be, 2); >>>>>> - if (ret < 0) { >>>>>> - dev_err(&dev->intf->dev, >>>>>> - "couldn't read from i2c device 0x%02x: error %i\n", >>>>>> - client->addr << 1, ret); >>>>>> - continue; >>>>>> - } >>>>>> - id = be16_to_cpu(id_be); >>>>>> + id = swab16(ret); /* LE -> BE */ >>>>> That's wrong! You can't assume that CPU is BE, as some archs use LE. >>>>> >>>>> You should, instead, call le16_to_cpu(), to be sure that it will be >>>>> doing the right thing. >>>>> >>>>> Something like: >>>>> >>>>> id = le16_to_cpu((__le16)ret); >>>> SMBus read/write word transfers are always LE (see SMBus spec section >>>> 6.5.5), >>>> which is also what i2c_smbus_xfer_emulated() assumes: >>>> http://lxr.free-electrons.com/source/drivers/i2c/i2c-core.c#L3485 >>> I got that part, but, if the CPU is also LE, doing swab16() is >>> wrong. It should swap it *only* if the CPU is BE. >> No, it should always be swapped, because the bytes are always transfered >> in the wrong order. >> The cpu endianess doesn't matter, (0x12 << 8) | 0x34 is always 0x1234. > You still didn't get it. > > Let's assume that the ID is 0x148c (MT9M112). > > This value, represented in low endian, is stored in memory as: > > unsigned char __id[2] = { 0x8c, 0x14 }; > > If we do: > u16 ret = *(u16 *)__id; > > What's stored at "ret" will depend if the sistem is LE or BE: > > on LE, ret == 0x148c > on BE, ret == 0x8c14 > > If you do: > u16 id = swapb16(val) > > you'll get: > > on LE, id == 0x8c14 > on BE, id == 0x148c > > So, the value will be *wrong* at LE. > > However, if you do: > id = le16_to_cpu((__le16)ret); > > On LE, this will evaluate to id = ret, and on BE, to id = swab16(ret). > So, on both, you'll have: > id = 0x148c. Can you please show me the code line(s) that make the value of the word returned by i2c_smbus_read_word_data() cpu endianess dependent ? :) Cheers, Frank > > > Thanks, > Mauro