[Outreachy kernel] [PATCH] gpu: drm: Use list_{next/prev}_entry instead of list_entry

[Julia Lawall]

On Mon, 19 Mar 2018, Arushi Singhal wrote:

> This patch replace list_entry with list_{next/prev}_entry as it makes
> the code more clear to read.
> Done using coccinelle:
>
> @@
> expression e1;
> identifier e3;
> type t;
> @@
> (
> - list_entry(e1->e3.next,t,e3)
> + list_next_entry(e1,e3)
> |
> - list_entry(e1->e3.prev,t,e3)
> + list_prev_entry(e1,e3)
> )

This looks like a rule that could be nice for the Linux kernel in general,
because the code really is much simpler.

I would suggest to write the rule in a more robust way, as follows:

@@
identifier e3;
type t;
t *e1;
@@

(
- list_entry(e1->e3.next,t,e3)
+ list_next_entry(e1,e3)
|
- list_entry(e1->e3.prev,t,e3)
+ list_prev_entry(e1,e3)
)

@@
expression e1;
identifier e3;
@@

(
- list_entry(e1->e3.next,typeof(*e1),e3)
+ list_next_entry(e1,e3)
|
- list_entry(e1->e3.prev,typeof(*e1),e3)
+ list_prev_entry(e1,e3)

This checks that the type that is specified corresponds to the one on e1.
It could actually be that the call is getting the first element of a list,
from some different type, and coincidentally the two types have the same
field name for the list element.

Unfortunately, the second rule, with the typeof call, doesn't currently
work in Coccinelle, because the semantic patch language doesn't actually
support typeof, and thinks that it is a function call.  I will fix this.

To make a semantic patch for the kernel, you can try running spgen on the
above file and answer the questions that it asks.  You can find examples
in the coccinelle/scripts directory.  Just run

spgen foo.cocci

Then answer the questions.  Then run

spgen foo.cocci > foo_for_kernel.cocci

The second run will use the results of the first run to print the semantic
patch.  Let me know if you have any questions.  You can always adjust the
semantic patch that is generated by hand afterwards if needed.

julia


>
> Signed-off-by: Arushi Singhal <arushisinghal19971997 at gmail.com>
> ---
>  drivers/gpu/drm/drm_lease.c                    | 2 +-
>  drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index 1402c0e..4dcfb5f 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -340,7 +340,7 @@ static void _drm_lease_revoke(struct drm_master *top)
>  				break;
>
>  			/* Over */
> -			master = list_entry(master->lessee_list.next, struct drm_master, lessee_list);
> +			master = list_next_entry(master, lessee_list);
>  		}
>  	}
>  }
> diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
> index e4c8d31..81c3567 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
> @@ -134,7 +134,7 @@ nvkm_cstate_find_best(struct nvkm_clk *clk, struct nvkm_pstate *pstate,
>  			       nvkm_volt_map(volt, volt->max2_id, clk->temp));
>
>  	for (cstate = start; &cstate->head != &pstate->list;
> -	     cstate = list_entry(cstate->head.prev, typeof(*cstate), head)) {
> +	     cstate = list_prev_entry(cstate, head)) {
>  		if (nvkm_cstate_valid(clk, cstate, max_volt, clk->temp))
>  			break;
>  	}
> --
> 2.7.4
>
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