PCI: rcar-gen4: Limit Max_Read_Request_Size to 256 Bytes

R-Car Gen4 PCIe controller has a hardware limitation of 256 Bytes
Max_Payload_Size (MPS). PCIe specification indicates that the MPS must not
exceed minimum MPS of any element along the packet path.  The PCI core
already enforces this, but add a check to warn if MPS is ever set larger
than 256.

R-Car Gen4 Reference Manual, chapter 104.4.8 Usage notes for MRRS (Max Read
Request Size) states:

  Please set "Max Read Request Size" to 128 bytes or 256 bytes.
  If "Max Read Request Size" is set to anything other than the
  above, the transferred data will not match the expected value.

This limitation also seems to apply to devices doing DMA reads, i.e.,
issuing MRd TLPs.  This limitation can be triggered by using non-HMB NVMe
SSD with Max_Read_Request_Size 512 Bytes, for example Crucial P5 Plus.  Any
write to the SSD (MRd TLP issued by the SSD controller) longer than 256
Bytes wraps around at the 256 Byte boundary, and the same data are written
to the SSD starting at offset 0 and at 256 Bytes.

Limit Max_Read_Request_Size to at most 256 Bytes for each device connected
to this PCIe controller to avoid this behavior.

A non-HMB (Host Memory Buffer) NVMe SSD can be identified using the
following command. Affected SSD reports 'hmpre' field as 0:

  $ nvme id-ctrl /dev/nvme0 | grep hmpre
  hmpre     : 0

The symptom is a read from the SSD which wraps around at 256 Byte boundary.
The test for this symptom can be implemented by writing 512 Byte of random
data into the SSD and reading the data back. If the read back data repeat
after 256 Bytes, the device is affected.

  $ dd if=/dev/urandom of=/tmp/data.bin bs=256 count=2 ; \
    dd if=/tmp/data.bin of=/dev/nvme0n1 bs=256 count=2 ; \
    dd if=/dev/nvme0n1 bs=256 count=2 of=/tmp/readback.bin

Expected data:
  $ hexdump -vC /tmp/data.bin
  00000000  97 81 b7 3b 0e 38 2b 4d  a7 d3 e0 47 ff c2 4b ca
  00000010  c1 85 98 f0 4a ac 03 a0  3b ab f3 19 44 dd 06 8b
  ...
  00000100  7a ce 3c b2 e1 d5 d9 11  88 63 10 59 76 3c dc 32 <-- random
  00000110  72 32 2a 7d a3 e1 aa 13  7c da 58 a1 7b 21 11 50 <-- data

Faulty readback, collected without this change in place:
  $ hexdump -vC /tmp/readback.bin
  00000000  97 81 b7 3b 0e 38 2b 4d  a7 d3 e0 47 ff c2 4b ca <---.
  00000010  c1 85 98 f0 4a ac 03 a0  3b ab f3 19 44 dd 06 8b <-. |
  ...                                                          | |
  00000100  97 81 b7 3b 0e 38 2b 4d  a7 d3 e0 47 ff c2 4b ca <-:-+- repeated
  00000110  c1 85 98 f0 4a ac 03 a0  3b ab f3 19 44 dd 06 8b <-+--- data
       ^^^
        |
        '--- Repeat starts at offset 0x100 = 256 Bytes

Fixes: 0d0c551011df ("PCI: rcar-gen4: Add R-Car Gen4 PCIe controller support for host mode")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260821020636.111719-1-marek.vasut+renesas@mailbox.org

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