Review request to DRM Driver for Samsung SoC Exynos4210.

Dave Airlie airlied at
Tue Aug 30 02:46:08 PDT 2011

>> 2. I'm not sure why samsung_drm_fn_encoder exists, it looks like from
>> the crtc mode set functions you call the encoder mode set functions,
>> is it not possible to use the helper drm_crtc_helper_set_mode so that
>> the crtc mode set is called then the encoder ones without having the
>> explicit calls? If not please either document it or point at the
>> explaination I missed.
> For generic usage of crtc, we got only encoder has display controller(or
> H/Ws) specific codes.
> as you pointed out, I will check how we use drm_crtc_helper_set_mode instead
> of samsung_drm_fb_encoder because drm_crtc_helper_set_mode calls not only
> crtc but also encoder's callbacks and in addition, we faced with vblank
> issue. drm framework has only one irq handler but in embedded system at
> least Samsung SoC, display controller and hdmi have independent their own
> irq. so I got crtc has common vblank operations such as enable_vblank and
> disable_vblank and each encoder's function to be called in accordance with
> activated crtc. if you think this way is wrong or not good way then do you
> have any good ideas to resolve this issue without Samsung_drm_fn_encoder,
> the ideas that crtc could access to encoder.?

Ah yes I see the issue now, that is probably a good reason, we should
think of some way to improve the vblank code to better allow this,
though that can probably wait until the driver is in mainline.

>> 3. Not terribly urgent but is samsung the best name for this? what
>> happens if you bring out another chip, I also think there is a lot of
>> samsung_drm_ in function names that seems not that useful. dropping
>> _drm_ might help.
> As you know, Samsung SoCs  have a variety of prefixes such as s3c24xx,
> s3c64xx, s3c, s5p, s5pv210(=s5pc110) and exynos4210(=s5pc210, =s5pv310).
> except old SoCs, our drm driver would support only exynos4210 and later. so
> we used samsung_ as prefix to represent them. but if you think this prefix
> is not proper name then we would try to consider other names. And _drm_
> would help to debug. for instance, we could know who is  current function's
> owner.

Generally, I'd prefer the name to be closer to the chip than the
manufacturer, so exynos4210 or something like that, I just worry
calling it samsung will lead to a later samsung2. So far the driver
hasn't much specific code in it, but that may change.

>> 4 ioctls: drm_samsung_gem_map_off needs explicit padding before the
>> 64-bit, drm_samsung_gem_mmap needs explicit padding before the
>> 64-bit,, though I'm not sure you need these ioctls all now that the
>> dumb interface is upstream,
> I am  afraid I don't understand what you mean fully. Could you please give
> me more details about "drm_samsung_gem_map_off needs explicit padding before
> the 64-bit, drm_samsung_gem_mmap needs explicit padding before the
> 64-bit,,"? as DRM, I am a novice. drm_samsung_gem_map_off is used to get
> offset that actually this is hash key that this key is used to get a gem
> object corresponding to it at drm_gem_mmap(). and the gem object is
> transferred to page fault handler(samsung_drm_gem_fault) with
> vma->vm_private_data containing it. At this time, page fault handler gets a
> gem object from vma object and then maps user space to physical memory.
> call patch to drm_gem_mmap is as the following : mmap system call ->
> samsung_drm_gem_mmap -> drm_gem_mmap
> if the comments above are wrong way then feel free to give me your advices
> and I will be pleased.

If you have an 32-bit followed by a 64-bit, on different platforms
that will end up aligned differently,
generally we just try to be explicit and add another 32-bit pad after
the first 32-bit. It might not matter on ARM (I'm not sure)
it also might not matter until you move from 32-bit ARM to 64-bit ARM,
but its best to deal with these early.

> Basically gem_dumb_* and gem_* are same operation. The difference between
> them is that gem_dumb_ needs framebuffer information such width, height and
> bpp to allocate buffer and gem_ needs only size. in case of gem_dumb_, size
> would be calculated at kernel side(at samsung_drm_gem_dumb_create). do you
> think it's better using only gem_dumb_? if so, I will remove
> gem_create_ioctl, gem_map_offset_ioctl and gem_mmap functions.

I think using the dumb ioctls initially is a good plan, esp as you
have no tiling or acceleration support, the idea
behind the dumb ioctls is to give splash screens and maybe write a
generic driver in the future that can just do sw rendering.

Like at some point I forsee you needing driver specific ioctls for
alloc/mapping, I'd just rather wait until you have some userspace
available to use them that we can validate them with.

>> what is usr_addr in the gem create ioctl for? this seems wrong, it
>> also looks too short for 64-bit addresses, but passing in userspace
>> addr to kernel is generally not a great plan.
> This is my mistake. I will remove usr_addr from drm_samsung_ge_create
> structure. this variable isn't used anywhere.


>> 5. you probably don't need master_create/set ops.
> For h/w lock support, I think these operations should be used. If
> unnecessary, I will remove these operations. and could you please explain
> the purpose of drm_master?. I didn't understand the master fully. I'd try to
> find how we could use the master feature after understanding.

The drm lock is only for old pre-KMS DRI1 drivers, no modern driver
should be using it or initialising it. drm master is the process that
controls authorisation for direct rendering clients on the DRM device.
So if you have an X server + 3D app, the X server is a master, if you
switch users to another X server, that becomes the master. The driver
only really needs this stuff if handles direct rendering and has
command submission ioctls.

>> Dave.
> Thank yo for your comments again. it's been very helpful
> Best Regards
> Inki Dae.

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