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
date.
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.
The patchset is split into several sections:
- Patches 1-7 are minor patches which make esp.c checkpatch friendly, QOMify
ESPState,
and also fix up some trace events ready for later patches in the series
- Patches 8-13 unify the DMA transfer count. In particular there are 2 synthetic
variables dma_counter and dma_left within ESPState which do not need to
exist.
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
and
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,
patches
14-25 work to eliminate the separate PDMA variables pdma_start, pdma_cur,
pdma_len
and separate PDMA buffers PDMA and CMD. The PDMA position variables can be
replaced
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
DMA
requests.