View source for R-Car/Merging-MMC-block-requests

== Overview ==

Linux kernel v5.4 supports mering MMC block requests by using IOMMU feature.
If we enables the feature on R-Car Gen3, MMC read/write performance can be improved.

=== Without this feature ===

* Linux block layer cannot merge any bio.
* Since R-Car Gen3 SDHI's max_segs is 1, Linux block layer only prepare one sg.

=== With this feature ===
* Linux block layer can merge bio if filesystem data is aligned with IOMMU (e.g. 4KB page).
* Even if R-Car Gen3 SDHI's max_segs is 1, this feature can map continuous pages if the data is aligned.
** So, since overhead which issuing read/write commands can be reduced than without this feature, MMC read/write performance can be improved.

==== Remarks ====

* Even if we enable this feature, Linux block layer cannot merge bio if filesystem data is not aligned (e.g. ext4 with small partition and -J size=1).

== Related commits ==

<pre>
158a6d3 iommu/dma: add a new dma_map_ops of get_merge_boundary()
6ba9941 dma-mapping: introduce dma_get_merge_boundary()
38c38cb mmc: queue: use bigger segments if DMA MAP layer can merge the segments
45147fb block: add a helper function to merge the segments
</pre>

== How to enable the feature on R-Car Gen3? ==

=== Kernel configuration ===

We need to enable the following kernel configuration.

<pre>
CONFIG_MMC=y
CONFIG_MMC_BLOCK=y
CONFIG_MMC_SDHI=y
CONFIG_MMC_SDHI_INTERNAL_DMAC=y
CONFIG_IOMMU_SUPPORT=y
CONFIG_IPMMU_VMSA=y
</pre>

=== Modify a driver ===

We need to modify drivers/iommu/ipmmu-vmsa.c like below.

==== v5.8 or earlier ====

<pre>
static const char * const rcar_gen3_slave_whitelist[] = {
	"ee100000.sd",
	"ee120000.sd",
	"ee140000.sd",
	"ee160000.sd",
};
</pre>

==== v5.9 or later ====

<pre>
static const char * const rcar_gen3_slave_whitelist[] = {
	"ee100000.mmc",
	"ee120000.mmc",
	"ee140000.mmc",
	"ee160000.mmc",
};
</pre>

=== How to confirm? ===

After the kernel booted, we can confirm whether the feature is enabled via sysfs.

<pre>
# ls /sys/kernel/iommu_groups/0/devices/
ee100000.mmc  ee140000.mmc  ee160000.mmc
</pre>

== Performance measurement ==

The following table is a performance measurement on v5.1-rc6<ref>https://patchwork.kernel.org/cover/10918221/</ref>.

{|
!environment
!Sequential Output (KB/sec)
!Sequential Input (KB/sec)
|-
|H3 without this feature
|117,133
|118,682
|-
|H3 with this feature
|130,482
|195,727
|}

== Reference ==
<references />