What is the PCIe i.MX6 max payload size ?

[NicolasP]

Hi,

In the reference manual, it is said at chapter 48.4.4.1.1.2 : "Program your PCI Express system with a larger value of Max_Payload_Size without exceeding CX_MAX_MTU (128)."

I can't find any definition for CX_MAX_MTU but the number in parenthesis sugests that it's value is 128. Right ?

Does this means that the max payload size of PCIe packets is 128 bytes ? If true, this means that one can't trim the max payload size which min value is 128 bytes.

Nicolas

[Yuri · Accepted solution]

From Mark Middleton :

   There are several parameters which sound like they can be set by the user through registers,

but are actually parameters that the core design engineer sets when designing the core into silicon.

Unfortunately, that means that these parameters cannot be changed.

  Three parameters in particular are:

CX_MAX_MTU = 128

CC_SLV_MTU = 128

CC_MSTR_BURST_LEN = 16

   If these values are exceeded in the application software, then the core starts to decompose messages,

which will affect throughput. The reason the core will not work with larger values is because the buffers and

memory space needed to support the larger values are not built into the core.

[NicolasP]

Hi Yuri,

Thanks for your answer.

That's what I thought. One can not change max payload size. It is fixed to 128 bytes.

Regards,

Nicolas