On 12/18/2012 07:21 AM, Rob Clark wrote: > On Mon, Dec 17, 2012 at 11:04 PM, Dave Airlie<airlied at gmail.com> wrote: >> So this might be a bit off topic but this whole CDF triggered me >> looking at stuff I generally avoid: >> >> The biggest problem I'm having currently with the whole ARM graphics >> and output world is the proliferation of platform drivers for every >> little thing. The whole ordering of operations with respect to things >> like suspend/resume or dynamic power management is going to be a real >> nightmare if there are dependencies between the drivers. How do you >> enforce ordering of s/r operations between all the various components?
There have been already some ideas proposed to resolve this at the PM subsystem level [1]. And this problem is of course not only specific to platform drivers. The idea of having monolithic drivers, just because we can't get the suspend/resume sequences right otherwise, doesn't really sound appealing. SoC IPs get reused on multiple different SoC series, no only by single manufacturer. Whole graphics/video subsystems are composed from smaller blocks in SoCs, with various number of distinct sub-blocks and same sub-blocks repeated different number of times in a specific SoC revision. Expressing an IP as a platform device seems justified to me, often these platform devices have enough differences to treat them as such. E.g. belong in different power domain/use different clocks. Except there is big issue with the power management... However probably more important is to be able to have driver for a specific IP in a separate module. And this suspend/resume ordering issue is not only about the platform devices. E.g. camera subsystem can be composed of an image sensor sub-device driver, which is most often an I2C client driver, and of multiple SoC processing blocks. The image sensor can have dependencies on the SoC sub-blocks. So even if we created monolithic driver for the SoC part, there are still two pieces to get s/r ordering right - I2C client and SoC drivers. And please don't propose to merge the sensor sub-device driver too. There has been a lot of effort in V4L2 to separate those various functional blocks into sub-devices, so they can be freely reused, without reimplementing same functionality in each driver. BTW, there has been a nice talk about these topics at ELCE [2], particularly slide 22 is interesting. I believe the solution for these issues really needs to be sought in the PM subsystem itself. > I tend to think that sub-devices are useful just to have a way to > probe hw which may or may not be there, since on ARM we often don't > have any alternative.. but beyond that, suspend/resume, and other > life-cycle aspects, they should really be treated as all one device. > Especially to avoid undefined suspend/resume ordering. [1] https://lkml.org/lkml/2009/9/9/373 [2] http://elinux.org/images/9/90/ELCE2012-Modular-Graphics-on-Embedded-ARM.pdf Thanks, Sylwester
