Re: TTM questions

[Jerome Glisse <glisse@freedesktop.org>]

On 07/15/2010 11:54 AM, James Simmons wrote:
>
> 	Now that I'm adding in TTM support to my 3Dfx driver I have a questions.
> First I noticed in almost every driver for ttm initialization that two
> struct ttm_global_reference global_ref are allocated. One for TTM_GLOBAL_TTM_MEM
> and one for TTM_GLOBAL_TTM_BO. For a graphics card with only VRAM fb memory do
> you need to allocate the TTM_GLOBAL_TTM_BO area? From what I can tell that is yes.
> 	After you set that up do pretty much do a:
>

Yes you need to have the global stuff this are accounting stuff.

> ret = ttm_bo_device_init(&bdev, bo_global_ref.ref.object,
>                           &my_bo_driver, DRM_FILE_PAGE_OFFSET,
>                           dma_flag);
> if (ret) {
> 	DRM_ERROR("failed!!!\n");
>          return ret;
> }
>
> ret = ttm_bo_init_mm(bdev, TTM_PL_VRAM, vram_size>>  PAGE_SHIFT);
> if (ret) {
>          DRM_ERROR(dev, "failed\n");
>          return ret;
> }
>
> // vram start
> placement.fpfn = 0;
> // static fb pci region length
> placement.lpfn = pci_resource_len(pdev, num); // static fb pci region
> // Anything else needed in placement?
>

fpfn & lfpn are page frame number ie if you want to have your
buffer allocated between address 0x10000000 & 0x20000000 you
set :
placement.fpfn = 0x10000000 / 4096;
placement.lpfn = 0x20000000 / 4096;

> // What is the alignment for non-tile framebuffers?
> // Is this related to info->pixmap.buf_align
> page_align = PAGE_MASK;
>

Alignement is to force your buffer to be aligned on some address.
For instance some hw need that the 8 lower bits of the framebuffer
address to be zero to have this set:
page_align = 256; //(2^8)

> // Usually the driver wants to start at the very beginning of vram
> buffer_start = 0;
> ttm_buffer_object *bo = kmalloc(sizeof(*bo);
>
> ret = ttm_bo_init(&bdev,&bo, vram_size, TTM_PL_VRAM,
>                   &placement, align, buffer_start, false, NULL,
>                   vram_size, my_bo_del_ttm);
> if (ret) {
> 	// ttm will call my_bo_del_ttm if it fails..
>          return ret;
> }
>
> // Is the below needed ?
> ret = ttm_bo_reserve(bo, false, false, false, 0);
> if (ret)
>          return ret;
>
> ret = ttm_bo_validate(bo,&nvbo->placement, false, false, false);
>
> ttm_bo_unreserve(bo);
>
>
> Please fill in what I'm missing.
>

bo_reserve/unreserve is for reserving a bo as multiple context might
access same bo. By reserving/unreserving you assure you are the only
one accessing the bo.

ttm_bo_validate is used when you want the gpu to do some work with the
bo. You need the memory manager to place the bo into visible gpu ram
and that's what ttm_bo_validate do. Once a bo is no more needed ttm
might move it out of vram in your back.

Hope it helps.

Cheers,
Jerome