On 05/23/2015 09:27 AM, Guenter Roeck wrote: > On Thu, May 21, 2015 at 02:07:31PM -0700, York Sun wrote: >> Lee, >> >> Is there any convention regarding I2C class bitmask? I see only three are >> defined for 3.12.19 and four for 4.0 >> >> I2C_CLASS_HWMON, I2C_CLASS_DDC, I2C_CLASS_SPD, I2C_CLASS_DEPRECATED >> >> I am working on a clock chip driver (SI5338) and trying to detect them (multiple >> chips in i2c mux). It would be a lot easier to have its own class, like >> I2C_CLASS_CLOCK. It is trivial to add a line to i2c.h file. Just checking if >> this is a bad idea. >> > > A class is supposed to indicate if a specific chip class is likely to be seen > on an i2c adapter, and that it may be necessary to auto-detect it (an example > are I2C_CLASS_HWMON type devices on PCs). The tendency, though, is to drop > existing markers for I2C_CLASS_xxx from adapter drivers as much as possible > because it slows down the boot process (see upstream commit 0c176170089c3). > > Auto-detection (with the _detect function) is not a preferred means to > instantiate a device. It takes time, and it is more or less unreliable. > For some chips, a read on its i2c register space can result in a chip reset, > or it can cause it to lose its programming. Worst case it can turn a system > into a brick. > > Preferred instantiations are listed in Documentation/i2c/instantiating-devices. > Instantiation with devicetree, ACPI, or through i2c_register_board_info() > would probably be the best available methods to instantiate a clock chip. > > Given that, first question is why you would want to have the chip auto-detected > in the first place. Is there any reason to believe that explicit instantiation > would not work in your system ? What are those reasons ? > > On top of that, the SI5338 does not have a clean way to detect the chip. > It does not have a chip ID register, and it is multi-banked. Given the > similarities of the various Silicon Labs clock chips, it may not even be > possible to reliably distinguish it from other SI chips. So even if you > had a good reason to auto-detect the chip, it would be _very_ unreliable. > This seems to be quite undesirable and risky for a clock chip. > Are you really sure that you want and need that ? > Guenter, Thanks for replying. No, I don't have to use autodetect. I was curious why there weren't more classes. I failed to notice which method is preferred in the mentioned document. I used to declare the devices by the bus number but met some issue when they are behind a mux. I temporarily used auto detect before I figure out how to describe the mux with i2c_board_info. Knowing auto-detection is not preferred, I will remove it from the proposed driver. Thanks again. York -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
