grist for the mill, that's

Something that initially seems bad or negative but is ultimately used in a positive way by someone. A: "The tabloids found out that you've been in rehab. How do you plan on handling it?" B: "It's just grist for the mill—I'm a changed man now, and that's what I'll tell the media. At least they're writing about me again!"

Fig. something useful or needed. Bob bases the novels he writes on his own experience, so everything that happens to him is grist for the mill. Ever since I started making patchwork quilts, every scrap of cloth I find is grist for the mill.

Something that can be used to advantage, as in These seemingly useless data will be grist for the mill when he lodges a complaint. This expression alludes to grist, the amount of grain that can be ground at one time. [Late 1500s]

COMMON If something is grist for the mill or grist to the mill, you can use it in a particular situation to help you to do something. Celebrity gossip is, of course, grist for the mill as far as the tabloids are concerned. You are, of course, much better at writing songs when you are completely miserable — it gives you so much more grist for the mill. Note: `Grist' was grain that was brought to a windmill or watermill to be ground. Millers needed regular supplies of grain to keep their businesses in operation.

That’s something useful, of which advantage can be taken. This metaphor refers to grist, the amount of grain to be ground at one time. It has been used figuratively since the sixteenth century. Arthur Golding, translator of Calvin’s theological writings, wrote, “There is no lykelihoode that those thinges will bring gryst to the mill” (1583). It was surely a cliché by the time Dickens wrote, “Meantime the fools bring grist to my mill, so let them live out their day” (Nicholas Nickleby, 1838).