Hi, this is just a rebase on top of latest linux-next. Follows the original blurb: Some device drivers need to sustain transfers to or from external memory at given fixed rates for a period of time. If we wait for the devfreq drivers to react to this load, the user is likely going to perceive glitches in the user experience. Some common scenarios that would benefit from it are display controllers setting a new mode, USB drivers starting isochronous transfers, camera drivers starting to capture and hw media codecs doing their thing. I'm proposing adding a new PM_QOS_MEMORY_BANDWIDTH class that device drivers can use to register their memory bandwidth needs right when they become known, so the external memory clock can be set at the optimum frequency, thus preventing glitches. There have been similar proposals before [0], the first main concern being that constraints need to refer to a global resource such as CPUs. If they referred to a network NIC or to a UART, there would be no way of knowing how to actually constrain each device because there would be a single list of requests. In this case, the memory bus is a global resource just like the CPUs are, and all requests in this class will apply to a single device driver in any particular system. The second serious concern was that constraints need to be defined in a way generic enough so they can be consumed in a wide array of particular systems without placing the burden of taking into account hw specifics in the driver of the memory client. Here, the PM_QOS_MEMORY_BANDWIDTH constraints are specified always in kbs, being the amount of data that the device driver wants to see transferred per second. This allows for example for a camera driver to calculate the appropriate value based on the size of each frame and the desired FPS and, depending on the particular system where the camera block is used, the appropriate external memory driver will convert that to the bus clock frequency that makes sense for the hardware at hand. The first patch adds the new class and also a new PM_QOS_SUM class type. The second illustrates how the driver for such a memory client would request its memory bandwidth requirement. Most vendor trees carry their own solution for this specific issue, so it would be pretty neat if mainline gained a similar mechanism so we get more power management goodies in the right place. [0] https://lkml.org/lkml/2012/3/10/165 Thanks, Tomeu Tomeu Vizoso (2): PM / QoS: Add PM_QOS_MEMORY_BANDWIDTH class drm/tegra: Request memory bandwidth for the display controller Documentation/power/pm_qos_interface.txt | 4 +++- drivers/gpu/drm/tegra/dc.c | 15 +++++++++++++++ drivers/gpu/drm/tegra/drm.h | 3 +++ include/linux/pm_qos.h | 5 ++++- kernel/power/qos.c | 27 ++++++++++++++++++++++++++- 5 files changed, 51 insertions(+), 3 deletions(-) -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
