SPI NOR Flash read speed too slow ~0.5MB/s

[Wang, Liugi]

Hi There,

I have a test setup running on Linux 4.19. The platform BIOS firmware is in a 64MB Micron Tech MT25QL512A NOR flash device. 
It is listed as /dev/mtd0.

When copy content from /dev/mtd0 to /temp folder,  it takes a long time to finish.
root at lab:~/tmp# dd if=/dev/mtd0 of=test1 bs=4096 count=8192
8192+0 records in
8192+0 records out
33554432 bytes (34 MB, 32 MiB) copied, 62.6115 s, 536 kB/s

root at lab:~/tmp# dd if=/dev/mtd0 of=test1 bs=16777216 count=2   
2+0 records in
2+0 records out
33554432 bytes (34 MB, 32 MiB) copied, 62.4701 s, 537 kB/s

I also tried QUAD mode and different SPI clock frequency 48MHZ, 24MHz. But I still got the similar read speed ~0.5MB/s.

Further debugging with an oscilloscope, it shows that for every read period of ~8us, there is an idle/delay interval about 120us.
This could explain why the averaged read speed was low.

We don't know where this long idle/delay period coming from and how to reduce it. 
The read period is very short ~8us and the delay interval 120us is too long. 

Is this kernel issue or SPI NOR driver issue? Or, something else?

Thanks

Liugi