Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> writes:
> From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
>
> Skip searching the software IO TLB if a device has never used it, making
> sure these devices are not affected by the introduction of multiple IO TLB
> memory pools.
>
> Additional memory barrier is required to ensure that the new value of the
> flag is visible to other CPUs after mapping a new bounce buffer. For
> efficiency, the flag check should be inlined, and then the memory barrier
> must be moved to is_swiotlb_buffer(). However, it can replace the existing
> barrier in swiotlb_find_pool(), because all callers use is_swiotlb_buffer()
> first to verify that the buffer address belongs to the software IO TLB.
>
> Signed-off-by: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
> ---
Excuse me if this is a silly question, but I'm not able to figure it out
on my own...
> include/linux/device.h | 2 ++
> include/linux/swiotlb.h | 7 ++++++-
> kernel/dma/swiotlb.c | 14 ++++++--------
> 3 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 5fd89c9d005c..6fc808d22bfd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -628,6 +628,7 @@ struct device_physical_location {
> * @dma_io_tlb_mem: Software IO TLB allocator. Not for driver use.
> * @dma_io_tlb_pools: List of transient swiotlb memory pools.
> * @dma_io_tlb_lock: Protects changes to the list of active pools.
> + * @dma_uses_io_tlb: %true if device has used the software IO TLB.
> * @archdata: For arch-specific additions.
> * @of_node: Associated device tree node.
> * @fwnode: Associated device node supplied by platform firmware.
> @@ -737,6 +738,7 @@ struct device {
> #ifdef CONFIG_SWIOTLB_DYNAMIC
> struct list_head dma_io_tlb_pools;
> spinlock_t dma_io_tlb_lock;
> + bool dma_uses_io_tlb;
You add this new member here, fine...
> #endif
> /* arch specific additions */
> struct dev_archdata archdata;
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 8371c92a0271..b4536626f8ff 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -172,8 +172,13 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
> if (!mem)
> return false;
>
> - if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC))
> + if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC)) {
> + /* Pairs with smp_wmb() in swiotlb_find_slots() and
> + * swiotlb_dyn_alloc(), which modify the RCU lists.
> + */
> + smp_rmb();
> return swiotlb_find_pool(dev, paddr);
> + }
> return paddr >= mem->defpool.start && paddr < mem->defpool.end;
> }
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index adf80dec42d7..d7eac84f975b 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -730,7 +730,7 @@ static void swiotlb_dyn_alloc(struct work_struct *work)
>
> add_mem_pool(mem, pool);
>
> - /* Pairs with smp_rmb() in swiotlb_find_pool(). */
> + /* Pairs with smp_rmb() in is_swiotlb_buffer(). */
> smp_wmb();
> }
>
> @@ -764,11 +764,6 @@ struct io_tlb_pool *swiotlb_find_pool(struct device *dev, phys_addr_t paddr)
> struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
> struct io_tlb_pool *pool;
>
> - /* Pairs with smp_wmb() in swiotlb_find_slots() and
> - * swiotlb_dyn_alloc(), which modify the RCU lists.
> - */
> - smp_rmb();
> -
> rcu_read_lock();
> list_for_each_entry_rcu(pool, &mem->pools, node) {
> if (paddr >= pool->start && paddr < pool->end)
> @@ -813,6 +808,7 @@ void swiotlb_dev_init(struct device *dev)
> #ifdef CONFIG_SWIOTLB_DYNAMIC
> INIT_LIST_HEAD(&dev->dma_io_tlb_pools);
> spin_lock_init(&dev->dma_io_tlb_lock);
> + dev->dma_uses_io_tlb = false;
...here you initialize it, fine...
> #endif
> }
>
> @@ -1157,9 +1153,11 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
> list_add_rcu(&pool->node, &dev->dma_io_tlb_pools);
> spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
>
> - /* Pairs with smp_rmb() in swiotlb_find_pool(). */
> - smp_wmb();
> found:
> + dev->dma_uses_io_tlb = true;
> + /* Pairs with smp_rmb() in is_swiotlb_buffer() */
> + smp_wmb();
> +
...and here you set it if swiotlb is used.
But, as far as I can tell, you don't actually *use* this field anywhere.
What am I missing?
Thanks,
jon
