[PATCH 0/1][RFC] drm/ttm Improved io_mem_reserve / io_mem_free_calling

Jerome Glisse j.glisse at gmail.com
Thu Nov 11 07:27:42 PST 2010

On Thu, Nov 11, 2010 at 3:41 AM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> The following patch is really intended for the next merge window.
> RFC:
> 1) Are there any implementations of driver::io_mem_reserve today that can't use
> the fastpath?
> 2) Can we put an atomic requirement on driver::io_mem_reserve /
> driver::io_mem_free?
> The patch improves on the io_mem_reserve / io_mem_free calling sequences by
> introducing a fastpath and an optional eviction mechanism.
> The fastpath is enabled by default and is switched off by the driver setting
> struct ttm_mem_type_manager::io_reserve_fastpath to false on mem type init.
> With the fastpath no locking occurs, and io_mem_free is never called.
> I'm not sure if there are any implementations today that could not use the
> fastpath.
> As mentioned in the patch, if the fastpath is disabled, calls to
> io_mem_reserve and io_mem_free are exactly balanced, and refcounted within
> struct ttm_mem_reg so that io_mem_reserve should never be called recursively
> for the same struct ttm_mem_reg.
> Locking is required to make sure that ptes are never present on when the
> underlying memory region is not reserved. Currently I'm using
> man::io_reserve_mutex for this. Can we use a spinlock? That would require
> io_mem_reserve and io_mem_free to be atomic.
> Optionally, there is an eviction mechanism that is activated by setting
> struct ttm_mem_type_manager::use_io_reserve_lru to true when initialized.
> If the eviction mechanism is activated, and io_mem_reserve returns -EAGAIN,
> it will attempt to kill user-space mappings to free up reserved regions.
> Kernel mappings (ttm_bo_kmap) are not affected.

Radeon can use fast path, i think nouveau can too. I am not sure we
can consider io_mem_reserve as atomic. Use case i fear is GPU with
remappable apperture i don't know what kind of code we would need for
that and it might sleep. Thought my first guess is that it likely can
be done atomicly.

Quick review of the patch looks good, i will try to take a closer look latter.

Jerome Glisse

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