front-end doesn't support SDMA or the latter implements a platform-
specific transfer method instead. While at it, factor out allocation
and freeing of SDMA resources to sdhci_dma_{alloc,free}() in order to
keep the code more readable when adding support for ADMA variants.
o Base the size of the SDMA bounce buffer on MAXPHYS up to the maximum
of 512 KiB instead of using a fixed 4-KiB-buffer. With the default
MAXPHYS of 128 KiB and depending on the controller and medium, this
reduces the number of SDHCI interrupts by a factor of ~16 to ~32 on
sequential reads while an increase of throughput of up to ~84 % was
seen.
Front-ends for broken controllers that only support an SDMA buffer
boundary of a specific size may set SDHCI_QUIRK_BROKEN_SDMA_BOUNDARY
and supply a size via struct sdhci_slot. According to Linux, only
Qualcomm MSM-type SDHCI controllers are affected by this, though.
Requested by: Shreyank Amartya (unconditional bump to 512 KiB)
o Introduce a SDHCI_DEPEND macro for specifying the dependency of the
front-end modules on the sdhci(4) one and bump the module version
of sdhci(4) to 2 via an also newly introduced SDHCI_VERSION in order
to ensure that all components are in sync WRT struct sdhci_slot.
o In sdhci(4):
- Make pointers const were applicable,
- replace a few device_printf(9) calls with slot_printf() for
consistency, and
- sync some local functions with their prototypes WRT static.
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