On Mon, Oct 27, 2014 at 04:32:34PM +0100, Thomas Petazzoni wrote:
> Enabling the hardware I/O coherency on Armada 370 and Armada XP
> requires a certain number of conditions:
>
> - On Armada 370, the cache policy must be set to write-allocate.
> - On Armada XP, the cache policy must be set to write-allocate, the
> pages must be mapped with the shareable attribute, and the SMP bit
> must be set
>
> Currently, on Armada XP, when CONFIG_SMP is enabled, those conditions
> are met. However, when Armada XP is used in a !CONFIG_SMP kernel, none
> of these conditions are met. With Armada 370, the situation is worse:
> since the processor is single core, regardless of whether CONFIG_SMP
> or !CONFIG_SMP is used, the cache policy will be set to write-back by
> the kernel and not write-allocate.
>
> Since solving this problem turns out to be quite complicated, and we
> don't want to let users with a mainline kernel known to have
> infrequent but existing data corruptions, this commit proposes to
> simply disable hardware I/O coherency in situations where it is known
> not to work.
>
> And basically, the is_smp() function of the kernel tells us whether it
> is OK to enable hardware I/O coherency or not, so this commit slightly
> refactors the coherency_type() function to return
> COHERENCY_FABRIC_TYPE_NONE when is_smp() is false, or the appropriate
> type of the coherency fabric in the other case.
>
> Thanks to this, the I/O coherency fabric will no longer be used at all
> in !CONFIG_SMP configurations. It will continue to be used in
> CONFIG_SMP configurations on Armada XP, Armada 375 and Armada 38x
> (which are multiple cores processors), but will no longer be used on
> Armada 370 (which is a single core processor).
>
> In the process, it simplifies the implementation of the
> coherency_type() function, and adds a missing call to of_node_put().
>
> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxxxxxxxxx>
> Fixes: e60304f8cb7bb545e79fe62d9b9762460c254ec2 ("arm: mvebu: Add hardware I/O Coherency support")
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.8+
> ---
> arch/arm/mach-mvebu/coherency.c | 37 +++++++++++++++++++++++--------------
> 1 file changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm/mach-mvebu/coherency.c b/arch/arm/mach-mvebu/coherency.c
> index 2bdc323..abf4535 100644
> --- a/arch/arm/mach-mvebu/coherency.c
> +++ b/arch/arm/mach-mvebu/coherency.c
> @@ -361,25 +361,34 @@ static int coherency_type(void)
> {
> struct device_node *np;
> const struct of_device_id *match;
> + int type;
>
> - np = of_find_matching_node_and_match(NULL, of_coherency_table, &match);
> - if (np) {
> - int type = (int) match->data;
> + /*
> + * The coherency fabric is needed:
> + * - For coherency between processors on Armada XP, so only
> + * when SMP is enabled.
> + * - For coherency between the processor and I/O devices, but
> + * this coherency requires many pre-requisites (write
> + * allocate cache policy, shareable pages, SMP bit set) that
> + * are only meant in SMP situations.
> + *
> + * Note that this means that on Armada 370, there is currently
> + * no way to use hardware I/O coherency, because even when
> + * CONFIG_SMP is enabled, is_smp() returns false due to the
> + * Armada 370 being a single-core processor.
Could we extend this a bit to let future readers know what needs to
change to re-enable support on the 370?
thx,
Jason.
> + */
> + if (!is_smp())
> + return COHERENCY_FABRIC_TYPE_NONE;
>
> - /* Armada 370/XP coherency works in both UP and SMP */
> - if (type == COHERENCY_FABRIC_TYPE_ARMADA_370_XP)
> - return type;
> + np = of_find_matching_node_and_match(NULL, of_coherency_table, &match);
> + if (!np)
> + return COHERENCY_FABRIC_TYPE_NONE;
>
> - /* Armada 375 coherency works only on SMP */
> - else if (type == COHERENCY_FABRIC_TYPE_ARMADA_375 && is_smp())
> - return type;
> + type = (int) match->data;
>
> - /* Armada 380 coherency works only on SMP */
> - else if (type == COHERENCY_FABRIC_TYPE_ARMADA_380 && is_smp())
> - return type;
> - }
> + of_node_put(np);
>
> - return COHERENCY_FABRIC_TYPE_NONE;
> + return type;
> }
>
> int coherency_available(void)
> --
> 2.0.0
>
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