Re: [Qemu-devel] [PATCH 3/3] ide: really restart pending and in-flight atapi dma

[Eric Blake]

On 04/06/2016 12:40 AM, Denis V. Lunev wrote:
> From: Pavel Butsykin <address@hidden>
> 
> Restart of ATAPI DMA used to be unreachable, because the request to do
> so wasn't indicated in bus->error_status due to the lack of spare bits, and
> ide_restart_bh() would return early doing nothing.
> 
> This patch makes use of the observation that not all bit combinations were
> possible in ->error_status. In particular, IDE_RETRY_READ only made sense
> together with IDE_RETRY_DMA or IDE_RETRY_PIO. This allows to re-use
> IDE_RETRY_READ alone as an indicator of ATAPI DMA restart request.
> 
> To makes things more uniform, ATAPI DMA gets its own value for ->dma_cmd.
> As a means against confusion, macros are added to test the state of
> ->error_status.
> 
> The patch fixes the restart of both in-flight and pending ATAPI DMA,
> following the scheme similar to that of IDE DMA.
> 
> Signed-off-by: Pavel Butsykin <address@hidden>
> Signed-off-by: Denis V. Lunev <address@hidden>
> ---

I'll leave the technical feasibility of this to others, but have some
coding style comments:


> @@ -783,8 +782,10 @@ static int ide_handle_rw_error(IDEState *s, int error, 
> int op)
>          s->bus->error_status = op;
>      } else if (action == BLOCK_ERROR_ACTION_REPORT) {
>          block_acct_failed(blk_get_stats(s->blk), &s->acct);
> -        if (op & IDE_RETRY_DMA) {
> +        if (IS_IDE_RETRY_DMA(op)) {
>              ide_dma_error(s);

I'd probably have split this into two patches; one adding the accessor
macros for existing access, and the other adding the new bit pattern
(mixing a conversion along with new code is a bit trickier to review in
one patch).


> +++ b/hw/ide/internal.h
> @@ -338,6 +338,7 @@ enum ide_dma_cmd {
>      IDE_DMA_READ,
>      IDE_DMA_WRITE,
>      IDE_DMA_TRIM,
> +    IDE_DMA_ATAPI

Please keep the trailing comma, so that the next addition to this enum
won't have to adjust an existing line.

>  };
>  
>  #define ide_cmd_is_read(s) \
> @@ -508,11 +509,27 @@ struct IDEDevice {
>  /* These are used for the error_status field of IDEBus */
>  #define IDE_RETRY_DMA  0x08
>  #define IDE_RETRY_PIO  0x10
> +#define IDE_RETRY_ATAPI 0x20 /* reused IDE_RETRY_READ bit */
>  #define IDE_RETRY_READ  0x20
>  #define IDE_RETRY_FLUSH 0x40
>  #define IDE_RETRY_TRIM 0x80
>  #define IDE_RETRY_HBA  0x100

Seems rather sparse on the comments about which bit patterns are valid
together.  If IDE_RETRY_READ is always used with at least one other bit,
it might make more sense to have an IDE_RETRY_MASK that selects the set
of bits being multiplexed, and/or macros that define the bits used in
combination.  Something along the lines of:

#define IDE_RETRY_MASK        0x38
#define IDE_RETRY_READ_DMA    0x28
#define IDE_RETRY_READ_PIO    0x30
#define IDE_RETRY_ATAPI       0x20

>  
> +#define IS_IDE_RETRY_DMA(_status) \
> +    ((_status) & IDE_RETRY_DMA)
> +
> +#define IS_IDE_RETRY_PIO(_status) \
> +    ((_status) & IDE_RETRY_PIO)

and these seem prone to false positives; where it might be better to do:

#define IS_IDE_RETRY_DMA(_status) \
    (((_status) & IDE_RETRY_MASK) == IDE_RETRY_READ_DMA)

> +
> +/*
> + * The method of the IDE_RETRY_ATAPI determination is to use a previously
> + * impossible bit combination as a new status value.
> + */
> +#define IS_IDE_RETRY_ATAPI(_status)   \
> +    (((_status) & IDE_RETRY_ATAPI) && \
> +     !IS_IDE_RETRY_DMA(_status) &&    \
> +     !IS_IDE_RETRY_PIO(_status))
> +

And this evaluates _status more than once, compared to:

#define IS_IDE_RETRY_ATAPI(_status) \
    (((_status) & IDE_RETRY_MASK) == IDE_RETRY_ATAPI)


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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