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

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/11/2010 04:27 PM, Jerome Glisse wrote:
> On Thu, Nov 11, 2010 at 3:41 AM, Thomas Hellstrom<thellstrom@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.
>    

In that case, I think I will change it to a spinlock, with a code 
comment that it can be changed to a mutex later if it turns out to be 
very hard / impossible to implement atomic operations. Another possible 
concern is the execution of umap_mapping_range() that may in some cases 
be long. Perhaps too long to use a spinlock.

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

Great. Thanks,
Thomas