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Re: [PATCH 00/25] esp: consolidate PDMA transfer buffers

From: Paolo Bonzini
Subject: Re: [PATCH 00/25] esp: consolidate PDMA transfer buffers
Date: Wed, 13 Jan 2021 15:39:29 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0

On 30/12/20 16:37, Mark Cave-Ayland wrote:
This patch series comes from an experimental branch that I've been working on
to try and boot a MacOS toolbox ROM under the QEMU q800 machine. The effort is
far from complete, but it seems worth submitting these patches separately since
they are limited to the ESP device and form a substantial part of the work to

As part of Laurent's recent q800 work so-called PDMA (pseudo-DMA) support was
added to the ESP device. This is whereby the DREQ (DMA request) line is used
to signal to the host CPU that it can transfer data to/from the device over
the SCSI bus.

The existing PDMA tracks 4 separate transfer data sources as indicated by the
ESP pdma_origin variable: PDMA, TI, CMD and ASYNC with an independent variable
pdma_len to store the transfer length. This works well with Linux which uses a
single PDMA request to transfer a number of sectors in a single request.

Unfortunately the MacOS toolbox ROM has other ideas here: it sends data to the
ESP as a mixture of FIFO and PDMA transfers and then uses a mixture of the FIFO
and DMA counters to confirm that the correct number of bytes have been
transferred. For this to work correctly the PDMA buffers and separate pdma_len
transfer counter must be consolidated into the FIFO to allow mixing of both
types of transfer within a single request.

This is all esp.c, so if Laurent and you are fine just send a pull request.



The patchset is split into several sections:

- Patches 1-4 are minor patches which make esp.c checkpatch friendly whilst also
   fixing up some trace events ready for later patches in the series
- Patches 5-11 unify the DMA transfer count. In particular there are 2 synthetic
   variables dma_counter and dma_left within ESPState which do not need to 
   DMA transfer lengths are programmed into the TC (transfer count) register 
which is
   decremented for each byte transferred, generating an interrupt when it 
reaches zero.
   These patches add helper functions to read the TC and STC registers directly 
   remove these synthetic variables so that the DMA transfer length is now 
tracked in
   a single place.

- Now that the TC register represents the authoritative DMA transfer length, 
   12-20 work to eliminate the separate PDMA variables pdma_start, pdma_cur, 
   and separate PDMA buffers PDMA and CMD. The PDMA position variables can be 
   by the existing ESP cmdlen and ti_wptr/ti_rptr, whilst the FIFO (TI) buffer 
is used
   for incoming data with commands being accumulated in cmdbuf as per standard 

- Patches 21 and 22 fix the detection of missing SCSI targets by the MacOS 
toolbox ROM
   on startup at which point it will attempt to start reading information from 
   attached to the q800 machine.

- Patch 23 is the main rework of the PDMA buffer transfers: instead of tracking 
   SCSI transfers using a separate ASYNC pdma_origin, the contents of the 
   async_buf are copied to the FIFO buffer in 16-byte chunks with the transfer 
   and IRQs being set accordingly.

- Patch 24 removes the last separate PDMA variable pdma_origin, including the 
   PDMA migration subsection which is no longer required (see note below about 
- Finally patch 25 enables 4 byte PDMA reads/writes over the SCSI bus which are used
   by MacOS when reading the next stage bootloader from CDROM (this is an 
increase from
   2 bytes currently implemented and used by Linux).


I've tested this on my SPARC32 OpenBIOS images which include Linux, OpenBSD, 
and Solaris and all of these continue to boot as before.

Similarly the q800 m68k Linux test image still boots as before with these 
applied. It is possible with lots of hacks to load Laurent's EMILE bootloader 
a MacOS toolbox ROM - the hope is to try and start upstreaming more of these 
as time allows.


The patchset ensures that ESP devices without PDMA (i.e. everything except the 
machine) will migrate successfully. This is fairly simple: the only change 
here is to copy the old synthetic dma_left value over into the TC.

Unfortunately migrating the PDMA subsection is a lot harder due to the change 
in the
way that the DMA TC and changes to the point at which transfer counters are 
For this reason the patchset will not migrate from older q800 snapshots: I don't
believe this to be a problem since some devices are still missing 
plus there are likely to be more breaking changes as the q800 machine matures.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

Mark Cave-Ayland (25):
   esp: checkpatch fixes
   esp: add trace event when receiving a TI command
   esp: fix esp_reg_read() trace event
   esp: add PDMA trace events
   esp: determine transfer direction directly from SCSI phase
   esp: introduce esp_get_tc() and esp_set_tc()
   esp: introduce esp_get_stc()
   esp: apply transfer length adjustment when STC is zero at TC load time
   esp: remove dma_counter from ESPState
   esp: remove dma_left from ESPState
   esp: remove minlen restriction in handle_ti
   esp: introduce esp_pdma_read() and esp_pdma_write() functions
   esp: use pdma_origin directly in esp_pdma_read()/esp_pdma_write()
   esp: move pdma_len and TC logic into esp_pdma_read()/esp_pdma_write()
   esp: accumulate SCSI commands for PDMA transfers in cmdbuf instead of
   esp: remove redundant pdma_start from ESPState
   esp: move PDMA length adjustments into
   esp: use ti_wptr/ti_rptr to manage the current FIFO position for PDMA
   esp: use in-built TC to determine PDMA transfer length
   esp: remove CMD pdma_origin
   esp: rename get_cmd_cb() to esp_select()
   esp: fix PDMA target selection
   esp: use FIFO for PDMA transfers between initiator and device
   esp: remove pdma_origin from ESPState
   esp: add 4 byte PDMA read and write transfers

  hw/scsi/esp.c         | 461 +++++++++++++++++++++++++-----------------
  hw/scsi/trace-events  |   5 +
  include/hw/scsi/esp.h |  20 +-
  3 files changed, 279 insertions(+), 207 deletions(-)

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