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Re: [PATCH v2 11/42] esp: apply transfer length adjustment when STC is z

From: Mark Cave-Ayland
Subject: Re: [PATCH v2 11/42] esp: apply transfer length adjustment when STC is zero at TC load time
Date: Tue, 16 Feb 2021 21:52:40 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 16/02/2021 07:33, Philippe Mathieu-Daudé wrote:

On 2/9/21 8:29 PM, Mark Cave-Ayland wrote:
Perform the length adjustment whereby a value of 0 in the STC represents
a transfer length of 0x10000 at the point where the TC is loaded at the

0x10000 -> 64 KiB?

I'd prefer to keep these as they are, since TC is described in the documentation as 16-bit counter: it is the number of bits that is relevant here as opposed to the absolute size.

There is a slight bit of trickery here in that the ESP emulation already handles a later variant of the chip which has a 24-bit counter which is why we can get away with setting its value to 0x10000 - guests that don't check for this will simply ignore the register containing the MSB.

start of a DMA command rather than just when a TI (Transfer Information)
command is executed. This better matches the description as given in the

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
  hw/scsi/esp.c | 9 +++++----
  1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index a1acc2c9bd..02b7876394 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -562,9 +562,6 @@ static void handle_ti(ESPState *s)
dmalen = esp_get_tc(s);
-    if (dmalen == 0) {
-        dmalen = 0x10000;
-    }
      s->dma_counter = dmalen;
if (s->do_cmd) {
@@ -699,7 +696,11 @@ void esp_reg_write(ESPState *s, uint32_t saddr, uint64_t 
          if (val & CMD_DMA) {
              s->dma = 1;
              /* Reload DMA counter.  */
-            esp_set_tc(s, esp_get_stc(s));
+            if (esp_get_stc(s) == 0) {
+                esp_set_tc(s, 0x10000);

0x10000 -> 64 * KiB

And same here too.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>



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