Re: [RESEND PATCH v17 5/5] iommu/arm-smmu: Add support for qcom,smmu-v2 variant

On Sat, Nov 24, 2018 at 3:34 AM Will Deacon [off-list ref] wrote:

On Fri, Nov 23, 2018 at 03:06:29PM +0530, Vivek Gautam wrote:

quoted

On Fri, Nov 23, 2018 at 2:52 PM Tomasz Figa [off-list ref] wrote:

quoted

On Fri, Nov 23, 2018 at 6:13 PM Vivek Gautam
[off-list ref] wrote:

quoted

On Wed, Nov 21, 2018 at 11:09 PM Will Deacon [off-list ref] wrote:

quoted

On Fri, Nov 16, 2018 at 04:54:30PM +0530, Vivek Gautam wrote:

quoted

@@ -2026,6 +2027,17 @@ ARM_SMMU_MATCH_DATA(arm_mmu401, ARM_SMMU_V1_64K, GENERIC_SMMU);
 ARM_SMMU_MATCH_DATA(arm_mmu500, ARM_SMMU_V2, ARM_MMU500);
 ARM_SMMU_MATCH_DATA(cavium_smmuv2, ARM_SMMU_V2, CAVIUM_SMMUV2);

+static const char * const qcom_smmuv2_clks[] = {
+     "bus", "iface",
+};
+
+static const struct arm_smmu_match_data qcom_smmuv2 = {
+     .version = ARM_SMMU_V2,
+     .model = QCOM_SMMUV2,
+     .clks = qcom_smmuv2_clks,
+     .num_clks = ARRAY_SIZE(qcom_smmuv2_clks),
+};

These seems redundant if we go down the route proposed by Thor, where we
just pull all of the clocks out of the device-tree. In which case, why
do we need this match_data at all?

Which is better? Driver relying solely on the device tree to tell
which all clocks
are required to be enabled,
or, the driver deciding itself based on the platform's match data,
that it should
have X, Y, & Z clocks that should be supplied from the device tree.

The former would simplify the driver, but would also make it
impossible to spot mistakes in DT, which would ultimately surface out
as very hard to debug bugs (likely complete system lockups).

Thanks.
Yea, this is how I understand things presently. Relying on device tree
puts the things out of driver's control.

But it also has the undesirable effect of having to update the driver
code whenever we want to add support for a new SMMU implementation. If
we do this all in the DT, as Thor is trying to do, then older kernels
will work well with new hardware.

Fair enough, if you're okay with that. Obviously one would still have
to change the DT bindings to list the exact set of clocks for the new
hardware variant, unless the convention changed recently.

Best regards,
Tomasz