[OpenBIOS] Bug in ESP DMA implementation?

[Mark Cave-Ayland]

Hi all,

Continuing more with my work on migrating SPARC32 to OFMEM, I've hit an 
issue with the ESP driver which is causing me a bit of a problem.

At the moment, I have a hybrid old-mem/OFMEM SPARC32 setup where I am 
migrating the various memory calls over to OFMEM one at a time. 
Currently my implementation just uses OFMEM for allocating MMU page 
tables, and with the default compile option of -Os looks like this:

Configuration device id QEMU version 1 machine id 32
Unhandled Exception 0x0000001f
PC = 0xffd12f08 NPC = 0xffd12f0c
Stopping execution

The interesting part is that this problem goes away if I compile with 
any -O3 or -O0 or but shows when I compile with -O2, -O1 or -Os. So I 
wonder if I've hit some kind of logic bug in OpenBIOS?

Looking at the SPARCv8 specification, exception 0x1f is equivalent to 
IRQ15 and the offending code where the error occurs can be found in 
drivers/esp.c:do_command():


     esp->ll->regs[ESP_BUSID] = sd->id & 7;
     // Set DMA address
     esp->espdma.regs->st_addr = esp->buffer_dvma;
     // Set DMA length
     esp->ll->regs[ESP_TCLOW] = cmdlen & 0xff;
     esp->ll->regs[ESP_TCMED] = (cmdlen >> 8) & 0xff;
     // Set DMA direction and enable DMA
     esp->espdma.regs->cond_reg = DMA_ENABLE;

/* Crash occurs somewhere in this section... */

     // Set ATN, issue command
     esp->ll->regs[ESP_CMD] = ESP_CMD_SELA | ESP_CMD_DMA;
     // Wait for DMA to complete. Can this fail?
     while ((esp->espdma.regs->cond_reg & DMA_HNDL_INTR) == 0)

/* End of crash section */;

     // Check status
     status = esp->ll->regs[ESP_STATUS];
     // Clear interrupts to avoid guests seeing spurious interrupts
     (void)esp->ll->regs[ESP_INTRPT];


I notice from the code above there is an explicit comment that mentions 
clearing interrupts to prevent the guest from seeing them so I would 
have thought that this wouldn't be an issue? I've checked the espdma 
structures to ensure that they are marked volatile (or _volatile_) and 
this appears to be the case - so I'm a little bit stumped. Can anyone 
point me in the right direction or spot the mistake?


ATB,

Mark.

-- 
Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
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