Hi Cinly, Thank your for your observation. After a quick look at the code in batch.c, it seems that the the additional DMI NOP operation is really redundant, as you point out. I haven't analyzed the code in detail, though.
The reason for the (perhaps redundant) NOP operation might have been be to collect the status of the preceding DMI operation - op field of the "dmi" JTAG register. That is however just a guess, and as you noted, the data is not used anywhere. If you'd like to improve the code, you are welcome to post a patch to https://github.com/riscv/riscv-openocd It will later be propagated here. Regards, Jan On Thu, Jun 3, 2021 at 1:05 PM Ooi, Cinly <cinly....@intel.com> wrote: > Hi All > > Apology in advance if I am not explaining this correctly, and, as it is > very likely, I am mistaken. > > > > When gdb issues the “load <image>” instruction for RISCV 0.13 compliant > processor, and if riscv target use the program buffer route > (write_memory_progbuf() [riscv-013.c:3368) the riscv target uses batch > processing to write the binary image data. See while loop on > [riscv-013.c:3437] > > > > In the while loop, riscv target chop up the image data to 32/64 bit words > for writing. RISCV target batch this up, preparing each 32/64 bit into dmi > instruction using riscv_batch_add_dmi_read() [riscv-013.c:3492-3]. > > > > So far, so good. > > My question is, in riscv_batch_add_dmi_write() [batch.c:116], at > > > batch:c124: riscv_fill_dmi_nop_u64(batch->target, (char > *)field->in_value); > > we are actually writing assigning a value to field->in_value. This will > eventually result in DR scan doing a read. > > > > I believe this read is redundant, as we are not using the return value > from DR. So this is my question: why do this read? > > In my driver, this extra read from DR is causing delays. Hence, I am > gathering opinion on this to see whether can I do some refactoring to take > out this read. I also believe it is not as easy as setting > field->in_value=NULL in rsicv_fill_dmi_nop_u64(), as it might be used > elsewhere that actually require field->in_value. > > > > Many thanks > > Cinly > > > Quick link to source code: > - riscv-013.c > https://sourceforge.net/p/openocd/code/ci/master/tree/src/target/riscv/riscv-013.c > - batch.c > https://sourceforge.net/p/openocd/code/ci/master/tree/src/target/riscv/batch.c >
