On Sat, Oct 06, 2012 at 03:20:19PM +0200, Julia Lawall wrote:
> Hello,
>
> I am looking at introducing some macros for i2c_msg initialization,
> and Ryan Mallon suggested that sometimes it could be useful to at
> the same time replace explicit lengths with the size of the
> associated buffer. But in some cases the sizes are not the same.
> An example is as follows, in drivers/gpu/drm/i915/dvo_ch7xxx.c:
>
> static bool ch7xxx_readb(struct intel_dvo_device *dvo, int addr,
> uint8_t *ch)
> {
> struct ch7xxx_priv *ch7xxx = dvo->dev_priv;
> struct i2c_adapter *adapter = dvo->i2c_bus;
> u8 out_buf[2];
> u8 in_buf[2];
>
> struct i2c_msg msgs[] = {
> {
> .addr = dvo->slave_addr,
> .flags = 0,
> .len = 1,
> .buf = out_buf,
> },
> {
> .addr = dvo->slave_addr,
> .flags = I2C_M_RD,
> .len = 1,
> .buf = in_buf,
> }
> };
>
> out_buf[0] = addr;
> out_buf[1] = 0;
>
> if (i2c_transfer(adapter, msgs, 2) == 2) {
> *ch = in_buf[0];
> return true;
> };
>
> if (!ch7xxx->quiet) {
> DRM_DEBUG_KMS("Unable to read register 0x%02x from %s:%02x.\n",
> addr, adapter->name, dvo->slave_addr);
> }
> return false;
> }
>
> The buffers both have size 2, but only one byte is asked to be read
> or written. Is there any need for the buffers to have size 2 in
> this case?
Looks like the 2 byte buffer size is just copy&pasta from writeb. And
didn't find any other reson for it not being just 1 byte.
> Unrelatedly, is it correct that ch has type uint8_t and out_buf and
> in_buf have type u8?
drm/i915 is totally confused about the (u)int*_t vs (s|t)* types
unfortunately. I think nowadays we mostly stick to _t typedefs, but not
consistenly.
Yours, Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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