Re: [GIT PULL 18/20] lightnvm: pblk: handle case when mw_cunits equals to 0

[Javier Gonzalez <javier@cnexlabs.com>]

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> On 28 May 2018, at 10.58, Matias Bjørling <mb@lightnvm.io> wrote:
> 
> From: Marcin Dziegielewski <marcin.dziegielewski@intel.com>
> 
> Some devices can expose mw_cunits equal to 0, it can cause creation
> of too small write buffer and cause performance to drop on write
> workloads.
> 
> To handle that, we use the default value for MLC and beacause it
> covers both 1.2 and 2.0 OC specification, setting up mw_cunits
> in nvme_nvm_setup_12 function isn't longer necessary.
> 
> Signed-off-by: Marcin Dziegielewski <marcin.dziegielewski@intel.com>
> Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
> Signed-off-by: Matias Bjørling <mb@lightnvm.io>
> ---
> drivers/lightnvm/pblk-init.c | 10 +++++++++-
> drivers/nvme/host/lightnvm.c |  1 -
> 2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/lightnvm/pblk-init.c b/drivers/lightnvm/pblk-init.c
> index d65d2f972ccf..0f277744266b 100644
> --- a/drivers/lightnvm/pblk-init.c
> +++ b/drivers/lightnvm/pblk-init.c
> @@ -356,7 +356,15 @@ static int pblk_core_init(struct pblk *pblk)
> 	atomic64_set(&pblk->nr_flush, 0);
> 	pblk->nr_flush_rst = 0;
> 
> -	pblk->pgs_in_buffer = geo->mw_cunits * geo->all_luns;
> +	if (geo->mw_cunits) {
> +		pblk->pgs_in_buffer = geo->mw_cunits * geo->all_luns;
> +	} else {
> +		pblk->pgs_in_buffer = (geo->ws_opt << 3) * geo->all_luns;
> +		/*
> +		 * Some devices can expose mw_cunits equal to 0, so let's use
> +		 * here default safe value for MLC.
> +		 */
> +	}
> 
> 	pblk->min_write_pgs = geo->ws_opt * (geo->csecs / PAGE_SIZE);
> 	max_write_ppas = pblk->min_write_pgs * geo->all_luns;
> diff --git a/drivers/nvme/host/lightnvm.c b/drivers/nvme/host/lightnvm.c
> index 41279da799ed..c747792da915 100644
> --- a/drivers/nvme/host/lightnvm.c
> +++ b/drivers/nvme/host/lightnvm.c
> @@ -338,7 +338,6 @@ static int nvme_nvm_setup_12(struct nvme_nvm_id12 *id,
> 
> 	geo->ws_min = sec_per_pg;
> 	geo->ws_opt = sec_per_pg;
> -	geo->mw_cunits = geo->ws_opt << 3;	/* default to MLC safe values */
> 
> 	/* Do not impose values for maximum number of open blocks as it is
> 	 * unspecified in 1.2. Users of 1.2 must be aware of this and eventually
> --
> 2.11.0

By doing this, 1.2 future users (beyond pblk), will fail to have a valid
mw_cunits value. It's ok to deal with the 0 case in pblk, but I believe
that we should have the default value for 1.2 either way.

A more generic way of doing this would be to have a default value for
2.0 too, in case mw_cunits is reported as 0.

Javier


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